My wife noticed a metal rattling sound coming from the muffler when standing outside the car when I was pulling into our garage - I checked it out and it turns out that a metal rattling/slight scraping sound lasting about 2-3 seconds could be heard every every 10 seconds or so when our 2003 3L H6 VDC Subaru Outback is in drive.  What could be the cause?  It seems to only be coming from the muffler - can't trace the sound to anywhere else under the car.

Generally speaking, rattles like that are heat shields that are loose.

Thanks NorthWet, I just found this heat shield thread while you were responding: 

 

http://www.subaruoutback.org/forums/66-problems-maintenance/20206-classic-subaru-heat-shield-rattle-question-about-welding.html

 

Does the rattling come from one of the two shield pieces being loose?  If so, a few hose clamps should keep it snug.

 

Only issue is that I only hear it from the muffler, not from the heat shield - this might be understandable considering that the heat shield is sealed, and the only opening is through the muffler in the back.  

those mufflers do have an odd bypass flap in them so, I suppose that could cause a rattle.

Thanks 1LuckyTexan.  If it's the bypass flap, is that flap replaceable or repairable?  I can't hear it when I drive, only when staying still or driving forward very slowly in drive.  I'll first try clamping the heat shield with a metal plumber's clamp.

Thanks 1LuckyTexan.  If it's the bypass flap, is that flap replaceable or repairable?  I can't hear it when I drive, only when staying still or driving forward very slowly in drive.  I'll first try clamping the heat shield with a metal plumber's clamp.

 

You're not gonna like the answer but, to my knowledge, the entire muffler would need to be replaced. And, Subaru wants a fortune for it.

 

Probably, most folks would just go aftermarket and live with w'ever effect it had on mileage/performance. probably not large, but, in theory, that bypass alloys for better comb. chamber scavenging at low rpms and better flow at high rpms. Junkyard might be an option.

You're not gonna like the answer but, to my knowledge, the entire muffler would need to be replaced. And, Subaru wants a fortune for it.

Make darn certain though, before damning the muffler. I don't read about too many problems with it. Heat shields or hangers or some other issue would be much more likely.

Probably, most folks would just go aftermarket and live with w'ever effect it had on mileage/performance. probably not large, but, in theory, that bypass alloys for better comb. chamber scavenging at low rpms and better flow at high rpms. Junkyard might be an option.

^^^

 

I have no idea what I did wrong, hit quote instead of edit it looks like.

 

sorry