2008 Metallic Rattles at Specific RPM

[msweet]

I have a '08 Outback 2.5 manual with ~45000km on it. We started hearing a metallic rattling sound about 2 months ago and it has gotten increasingly worse over time. The rattle only occurs at specific engine rpm (1400 & 2800) and there is no link to vehicle speed, transmission gear, etc. just rpm.

I originally thought it was the doughnut gasket with the spring bolts between the catalytic converter and the muffler leaking. There was water dripping from the flange when I started the car up cold. I replaced it with an after market gasket / bolt kit. It didn't seem to solve the problem as there is still water dripping and the noise is getting worse. Interesting to note that the factory springs for the gasket were fully compressed when I broke / removed them. My next thought is to try a factory replacement kit as I have read that the aftermarket kits may not fit perfectly.

I have also looked at the heat shields and could not find any that appeared to be loose. The fact that rattles occur at multiples of engine rpms (1400 & 2800) implies that it might be a harmonic vibration frequency causing these to vibrate.

Any advice on potential solutions would be greatly appreciated!

[plain OM]

Although it's possible, I tend to doubt the rattle is coming from that exhaust joint, but it could be something in the exhaust system somewhere (which could be more related to engine speed).

Is it only when the engine is under load? Have you tried to reproduce the noise with the car stopped, or up on a lift, and the engine revved to 1400 rpm? This should make isolating the source far easier.

[1 Lucky Texan]

heat shields were my first thought so, I guess I'm just gonna lurk to see what it turns out to be.

[ScoobyRu]

I think it's the heat shields still. I couldn't tell that mine were loose, but I noticed that my car was missing a heat shield clamp after studying a diagram for the system. Must have rusted off.

I slapped a cheap hose clamp to it, and the problem is gone. The clamp that was missing is the one right where the tubing attaches to the manifold.

[msweet]

Thanks for everyones thoughts so far. I can reproduce the noise while on blocks with no load. There doesn't seem to be any change in the noise with temperature, engine load, speed, etc.

I originally thought the sound was due to a leaking exhaust joint because the sound was more of a growl. But over time the sound has become more metallic and sounds more like heat shields. I haven't been able to get someone to rev to 1400 rpm while I am under the car yet. That is my next step.

[1 Lucky Texan]

Originally Posted by msweet

Thanks for everyones thoughts so far. I can reproduce the noise while on blocks with no load. There doesn't seem to be any change in the noise with temperature, engine load, speed, etc.

I originally thought the sound was due to a leaking exhaust joint because the sound was more of a growl. But over time the sound has become more metallic and sounds more like heat shields. I haven't been able to get someone to rev to 1400 rpm while I am under the car yet. That is my next step.

just for quick diagnosis, you could throw some zip ties around the shields, get in and rev the car. Then remove the ties one-at-a-time to pin down which shield is the problem. I'd do it on a cold vehicle.

[Dogfather]

My heat shield was rattling at that rpm despite nothing apparently missing or broken or worn-looking. I reached under there while my wife gave it a little gas. I was able to move the whole thing up about a 1/4 in., stopping the rattle.
A hose clamp has kept it quiet since.

[DavidPeab]

I'm leaning towards heat shields as well. I had a heck of time finding which of mine were rattling, until I got it on the ramps and saw/felt that it was both of the shields around the exhaust pipes just aft of the manifolds. They looked fine and the space between them and the pipes was about 1/32 of an inch. Just enough to cause a vibration at certain RPMs.

[msweet]

It turned out to be heat shield rattle just aft of the catalytic converter. A very small amount of movement caused such a great noise. I used the tried and true hose clamp method and all is quiet. Thanks for all the responses!

[robd]

Originally Posted by msweet

It turned out to be heat shield rattle just aft of the catalytic converter. A very small amount of movement caused such a great noise. I used the tried and true hose clamp method and all is quiet. Thanks for all the responses!

First post, sorry to be bringing this thread back from the dead, but I did not want to start another thread about this problem. There appear to be many threads about loose heat shields already, so I picked the newest one .

My 2006 is having the same rattling coming from the heat shield around the pipe from the drivers side exhaust port to passenger side up-pipe. It looks to me like some kind of stainless steel wool material packed around the ends of the heat shield has compressed or fallen out. I searched for exhaust packing, and steel wool here and saw no results. I am wondering if this was a repair done by the dealer or mechanic in the past, or am I the first person to ever notice this factory installed packing? (We are the original owners)

Can this material be repacked or replaced?

Thanks

Rob