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Discussion Starter #1
I recently start noticing thump thump noise in the cabin of my wife's 2011 outback 2.5.
I couldn't quite hear it in the engine bay under the hood. I turned off ac and fan, radio. Parked.
Frequency is about 3-5 per second.
I'm decent mechanically, but I have no idea what could be making this noise. I used mechanic stethoscope on alternator, ps pump, ac compressor, and they sound ok.

Any pointer? I recorded the noise with hope someone recognize it. But I cant upload MP4. Will find a way later.

Thanks in advanced.
 

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I note it's in P (Park) in the video, so let's assume it's engine related.

Did you check the idler pulleys as well when you used the mechanic's stethoscope to check the alternator, PS pump, and AC compressor?

Did you look at the radiator fans?

Does anything change when any of the following is done?
  • electrical loading chaange (turn on the lights heater grids, seat heaters, and fan to test)
  • AC on or off
  • PS pump demand increase (put slight turning pressure on steering wheel with wheel blocked into curb)
And note your 2011 2.5i has a timing belt, which is just about due for a change (105 months / 105k miles, whichever comes first), so you might check the condition of that ASAP.
 

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When I'm trying to diagnose an engine related noise, I like to begin by taking the belt off and starting the car. Nothing bad is going to happen if you do this, as all that's on the belt system is the alternator, PS pump and compressor. With those three items, as well as the idler pulley and tensioner out of the picture, you will have a better idea of where the sound comes from. And with the belt off, you can check all of these items for wear and free play.
 

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Discussion Starter #5
Thanks for the replies!
the noise have morphed into a squeaking noise. almost like i have a family of mouse under my driver side dash.
and yes it was in park, so it wasn't driveline related.

I note it's in P (Park) in the video, so let's assume it's engine related.

Did you check the idler pulleys as well when you used the mechanic's stethoscope to check the alternator, PS pump, and AC compressor?

Did you look at the radiator fans?

Does anything change when any of the following is done?
  • electrical loading chaange (turn on the lights heater grids, seat heaters, and fan to test)
  • AC on or off
  • PS pump demand increase (put slight turning pressure on steering wheel with wheel blocked into curb)
And note your 2011 2.5i has a timing belt, which is just about due for a change (105 months / 105k miles, whichever comes first), so you might check the condition of that ASAP.
i did check the idler pulley, it sounded ok. all the pulleys sounded ok, i think it could be better, but given the mileage, it's ok.
i didn't specifically look at radiator fans, but they were both on (engine was hot and AC was on). i didn't notice any noise from them.
nothing changes when electrical loading changes.
no changes with AC on / off. i could've bet some money that it was the hvac fan since the noise seem to be from the inside of the cabin. under the driver side dash.
i haven't done PS pump test. i will next time.

i've competely ignored the timing belt thing. does it make thump (and now squeak) sound when it wears?
how many hours is replacing timing belt job? kindda give me an estimate on shop time, i rarely have time to wrench anymore.

When I'm trying to diagnose an engine related noise, I like to begin by taking the belt off and starting the car. Nothing bad is going to happen if you do this, as all that's on the belt system is the alternator, PS pump and compressor. With those three items, as well as the idler pulley and tensioner out of the picture, you will have a better idea of where the sound comes from. And with the belt off, you can check all of these items for wear and free play.
i haven't done this, but i did listened all the pulleys, including the idler pulleys.
if the noise is only heard in the cabin, not in the engine bay, could it be anything from the pulleys? most likely would be from another motor/belt driven thing in the cabin right?
 

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i've competely ignored the timing belt thing. does it make thump (and now squeak) sound when it wears?
If a part of the timing belt were loose or frayed and striking the timing belt cover, or an idler had a bearing that was worn or wobbly, it would be a sound much like any other belt in this condition. Unlike any other belt, however, failure of a timing belt is often catastrophic. But the main point is to check it if you haven't looked at it; it's probably due and may be on borrowed time.

how many hours is replacing timing belt job? kindda give me an estimate on shop time, i rarely have time to wrench anymore.
Maybe 5-6 hours of your own time and $175 in parts.

i haven't done this, but i did listened all the pulleys, including the idler pulleys.
if the noise is only heard in the cabin, not in the engine bay, could it be anything from the pulleys? most likely would be from another motor/belt driven thing in the cabin right?
The likely culprit inside the cabin is the HVAC motor. If that's turned off and you still hear it, then it's not that. There are no belt-driven systems inside the cabin, as far as I know - last one of these was probably a cassette player.
 

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here's the video. you'll need to turn up the volume. it works better on my phone than my pc.
https://espublic.s3.amazonaws.com/20190824_115652.mp4
I'm hearing two distinct sounds. One is very steady time-wise and seems to me to be engine related, and I would think should be apparent when listening in the engine compartment. To see if it's from the engine, it should be listen with the mechanic's stethoscope touching the engine block.

The other reminds me more of a cricket-type sound, i.e., few clicks followed by a variable silent period, then a few clicks again (not always the same number), and so on. Or described differently, it's somewhat like a ratchet wrench being rotated in reverse at random intervals.

What happens to the sound(s) if the engine rpm is raised from idle?

If the engine is stopped but the ignition left in the On position, is either sound still heard?

the noise have morphed into a squeaking noise. almost like i have a family of mouse under my driver side dash.
Another recording, under the same "parked" conditions, perhaps, might be recognized.
 

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Discussion Starter #8
If a part of the timing belt were loose or frayed and striking the timing belt cover, or an idler had a bearing that was worn or wobbly, it would be a sound much like any other belt in this condition. Unlike any other belt, however, failure of a timing belt is often catastrophic. But the main point is to check it if you haven't looked at it; it's probably due and may be on borrowed time.

Maybe 5-6 hours of your own time and $175 in parts.

The likely culprit inside the cabin is the HVAC motor. If that's turned off and you still hear it, then it's not that. There are no belt-driven systems inside the cabin, as far as I know - last one of these was probably a cassette player.
Thanks for the info on the timing belt. i'm currently out of town, but i'll check the belt asap.
i heard there could be a couple servo motors in the dash to control air blending between ac and outside? also air routing between the various AC vents?

I'm hearing two distinct sounds. One is very steady time-wise and seems to me to be engine related, and I would think should be apparent when listening in the engine compartment. To see if it's from the engine, it should be listen with the mechanic's stethoscope touching the engine block.

The other reminds me more of a cricket-type sound, i.e., few clicks followed by a variable silent period, then a few clicks again (not always the same number), and so on. Or described differently, it's somewhat like a ratchet wrench being rotated in reverse at random intervals.

What happens to the sound(s) if the engine rpm is raised from idle?

If the engine is stopped but the ignition left in the On position, is either sound still heard?

Another recording, under the same "parked" conditions, perhaps, might be recognized.
i think your ears are better than mine. as i didn't hear the 2 distinct sound. the squeak is certainly louder now.
there's no change on rpm change, as far as i can observe. does this rule out timing belt?
i'll try the engine stopped with ignition/acc on and report back.


new observation, the noise is typically not there upon engine start. it only start after car has been on for a little while. and it sometimes stop, for a few seconds, then start again.
 

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i think your ears are better than mine. as i didn't hear the 2 distinct sound.
Maybe I'm "hearing things". Perhaps someone else can confirm, one way or another.

If there's no change in the sound with the engine rpm raised, then it's less likely engine-related.

Your idea of HVAC control actuators is a possibility. I think they would be heard with the ignition at ON, but the engine not running (presuming the engine is turned off after the sound appears).
 

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Discussion Starter #10
just to update the thread. the noise is mostly gone since the air temp gone up.
for now, i'm hoping it's just air mixture motor related.
 
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