Strange humming noise, maybe rear diff?

[Cowardlyduck]

Hi all,

I recently took my 05 Outback 2.5i for a spin on some sand. Had a blast doing doughnuts and nearly getting bogged several times on some serious 4WD tracks. The Outback performed marvellously and didn't miss a beat.

One of the reasons I was ok to 'mistreat' her a little on sand was because I was due for a service anyway and thought any issues arising as a result of the sand would get ironed out in the service anyway. Before you say moron...this turned out to be true and the few minor issues (low oil, cv joints, wheel truing, etc) did get fixed.

One issue however didn't. I did point it out to the mechanics when getting it serviced, but apparently they couldn't hear it.

When driving at higher speeds 60kmh+ and the engine is at or around 2000rpm there is a humming noise that seems to increase in pitch with the RPMs. Once you hit 3000rpm+ the engine noise is louder so you can't really hear it any more. It's loudest at 2000rpm cause that's where it seems to resonate the rest of the car most.

I think it might be the rear diff, but not sure as I've never experienced something like this before.

For more clarification I'll answer the mandatory questions below.

A. When does the sound happen?

_ When engine is running-YES
_ When transmission is in some particular gear-No, all gears, but louder in 3rd+.
_ When turning-No
_ On throttle-Yes, louder at higher speeds.
_ When braking-No
_ Over bumps-No
_ In cold conditions-No
_ At speed - Yes

B. Where does the sound come from?
Can't tell, but could be the Drivetrain. Seems maybe the rear diff, but not sure.

C. Does the sound vary with some condition?
Engine & transmission rpm dependent. At higher speeds around 2000rpm.

D. Is there a vibration associated with the noise?
No

E. Does it affect drive-ability? how?
No



Thanks,

[grossgary]

highly unlikely to be your rear diff - simply check the fluid level there first or change it if it's never been done, taking note of the level. subaru rear diffs almost never fail.

*** i would have someone ride in the back and try to determine positively (or maybe you already know?) if it's center or left/right? if someone sits in the rear cargo area (maybe you while a friend drives that does not have a life insurance policy on you LOL), it's usually very obvious and helpful to know this. of course i mean for one simple drive to make the noise, not any kind of extended distance.

no noise when coasting then? (i'm assuming not since you say no noise at all when braking).

Originally Posted by Cowardlyduck

and the few minor issues (low oil, cv joints,

exactly what happened with the CV joints.

here in the US the aftermarket axle companies absolutely suck. it's common to get new or rebuilt axles that have issues. it's so common that if you were in the US my first guess would be that you replaced the axles with aftermarkets and that's your issues.

specifically usually the inner DOJ's will do exactly what you're describing but that could change depending what happened, how it was repaired, etc.

also - i've driven in sand before and if you got any sand in the axles and simply rebooted that might be an issue - sand will eat the joints up very quickly. probably about one of the worst things to drive in for CV's. they'll last 50,000 miles when busted and out of grease on normal clean roads. in sand they'll start making noise after one time in the sand and in less than 100 miles. i've done it before.

rear wheel bearing comes to mind as well but i haven't seen many of these newer rear wheel bearings fail. some 96-2000 rear bearings fail pretty often, but not so common on 2005's.

[serotta]

There is an extended warranty on the 2005 wheel bearings. It is good to 100,000 miles. Mine were whining/humming/singing at 70,000. Subaru replaced them free of charge.
Do a search (rear wheel bearings, etc) as there are several threads out there describing the problem.

[grossgary]

Originally Posted by serotta

There is an extended warranty on the 2005 wheel bearings. It is good to 100,000 miles.

nice hit! weird the 2000's-2004's have few issues, wonder what changed.

[porcupine73]

Originally Posted by grossgary

nice hit! weird the 2000's-2004's have few issues, wonder what changed.

Probably switched to a different Chinese foundry

You said you were doing donuts, if you were doing reverse donuts, I have heard people say it messed up the outer joints on their front axles somehow. 'borked' them they said?

[1 Lucky Texan]

maybe heat shields?

[Cowardlyduck]

Originally Posted by grossgary

no noise when coasting then? (i'm assuming not since you say no noise at all when braking).

exactly what happened with the CV joints.

Don't think there's any noise when coasting. Will try and see how it sounds at higher speeds.
CV joints were replaced under warranty as there were only just done within the last 12 months. Service was carried out at the Subaru dealership here in Sydney.

I don't think it's the bearings as it happens around 2000rpm consistently in higher gears, but not really below that rpm range. Again, the coasting at higher speeds test should confirm this.

Originally Posted by 1 Lucky Texan

maybe heat shields?

Interesting point. There has been a bit of vibration from the engine when flooring it since a couple of services ago. I was thinking it might be the heat shields coming a bit loose. Is it possible they could 'hum' at a certain rpm range as well?

Cheers,

[aftCG]

+1 on wheel bearings. Find the service bulletin, print it out and take it in. Make sure they'll stand by it before you let them do the job.

Saved me a ton of money.

Oh and the heat shields definitely have a resonant frequency that will cause them to buzz.

[Seabass]

Another +1 for the bearing. It should be easy to diagnose though. Just jack it up and check for play.

[grossgary]

i've seen heat shields cause "load" specific noises - only in certain conditions/speeds - but i wouldn't call that the norm, just have to check. wouldn't expect that with an 05 unless you were living in the rust belt in the US. LOL

Originally Posted by Seabass

Another +1 for the bearing. It should be easy to diagnose though. Just jack it up and check for play.

is that 05+ specific bearing symptoms? good to know as earlier legacy/outbacks often have bad bearings but no play. i use an IR temp gun instead because jacking it up is often a waste of time.