annoying rattling noise undercarriage

[LONO100]

hi all, it's been a while since ive posted since my 1997 legacy 2.5GT has been running like a champ. i recently developed a weird rattling noise on the underside of my ride.

 

it is very peculiar. it doesnt show up until you drive it a few miles. it rattles like a penny in a coke can and is consistent with whenever you step on the gas pedal. when the motor is running but in N, no rattling, but when you step on the gas, here comes the rattling noise.

 

it's been like this for a few months, and a few weeks ago, i took it to get smogged with the rattling noise, and the thing passed smog like a champ (california).

 

i have read that it can be caused by loose or rotting heat shields. i jacked the car up, and checked every shiled on every pipe and didnt find anything loose. i then had my wife turn the car on and step on the gas while i listened closely to zero in on the rattling.

 

the only thing i was sure of was that the rattling wasnt coming from the motor, but somewhere in the pipes. the problem is, i cant tell which pipe it is coming from.

 

has anyone run into this problem? i thought at first that the insides of my cat had fallen apart, but ifv that's the case, is it still possible to pass smog?

 

any help with this would be really appreciated. i would hate to tear the thing apart looking for this crazy annoying sound when it might be something very obvious that i didnt think about. thanks in advance everyone.

[Stryder]

My best friend's heat shield rattles like crazy when he gets on the gas until it warms up, that's what I'd say the culprit is. See if you can get a friend to look under for you.

[adam_m]

I had a problem similar to this on a Nissan Sentra once upon a time. It ended up being some of the catalyst monolith material that had broken loose and was rattling around.

 

My Legacy has a rattle on deceleration, but I know that one is from the middle hanger not being bolted in; I put UEL headers on and the bolt holes didn't line up correctly.

 

I would recommend getting under the car with a rubber mallet and tapping (gently) on your heat shields and pipes until you narrow it down. It could be a number of things, but the most common issue is the heat shields.

[RustyRuu]

this has happened with me as well. drove me crazy, began hearing it in my sleep. Caught me a case of the ****-its and ended up cutting the heat shields on my exhaust off with a grinder. no more rattle. i knew it was just one buy hitting it with a metal wrench i was able to pinpoint which one it was but thought if it happened to one i should just get rid of them all. if you did this though and it was a catalytic converter problem then that would suck and be more work.