Bad CAT? Bad Y-pipe? Bad Clamp?

[Bluefury99V6]

I started having a rattle from underneath the passengers side about 5 days ago and it has progressively gotton worse. I've done a search on the board and that narrowed it down to one of three things. One bad clamp, two bad CAT, three bad Y-pipe. My Firebird only has 35k miles on it. I haven't had a chance to put it up on jack stands to get a better look and narrow down the problem. Bare with me, my question is coming up.. It has been raining here in DFW yesterday and today. The noise starts after the car is warmed up or after about 3 min of running idle after being cold. The rattle isn't really noticable at idle but if I rev it slightly as it comes back to idle I can hear it. I can hear it real good at parking lot speeds and driving through the 2200-3000 rpm range. I can stick the clutch in and rev it and hear the rattle at highway speeds or on the street. I have a muffler and exhaust off a WS-6. Last night when driving out of the parking garage at work I heard the rattle real good with the echo and all. I got on the tollway and it started raining. It started to pour real good and I got off at my exit and the car rattle was gone. It poured real good the whole way home and I only heard the rattle as I backed the car back into my garage. This morning on the way to work it was pouring real good and I didn't hear the rattle at all either. Lately I have always been able to hear the rattle when I launch the car from a stop light. I got no rattle at all. Here is the question.. Could this narrow down my rattle to a bad CAT? All the water on the road splashing up and cooling it off causing me not to hear the rattle? Or am I way off and the fact that it is raining and not hearing the rattle is just a coincidence?

[GregWS6'01]

i dunno what to tell you.. I had the same problem with my baffle in my cat breaking...that caused my rattle, i found it to be from hitting it on speed bumps.......Get someone else to rev it and try to see if it's coming from the cat.
I could only hear the broken baffle when at idle or low rpm's. Never at highway speed. Now might be a good time to put a high-flow on :D

[Bluefury99V6]

I'll check the baffle. I don't hear the rattle at idle though just if I rev it from an idle and on the way back down to idle.

Any more ideas???

[GregWS6'01]

how loud is it? the cat can be weird and it not make noises all the time. I now mine didn't until it started to get worse and more broken up.....

[Bluefury99V6]

I had my wife take the car for a spin and I can hear it when she drives by. The rattle sound kind of echos out of the exhaust pipes. It is starting to get louder inside the car too.

[defrag]

might be teh heat shield on top of the cat rattling around

[TheBeast]

just get up under the car and follow the pipes from before the cat all the way to the tips and notice if there is any rubbing against other parts, if so break out your hammer

[knowledgefreak]

sounds like your cat is breaking up take it to the dealer quick lol before your warranty is up.

[96 T-Top Bird]

Alright, i've been having many rattle problems w/ my exhaust [img]smile.gif[/img]

In the morning when the car is COLD!, go underneith it and start banging on the y-pipe that goes from the driver side of the engine bay across and meets w/ the passenger side.

Hit that a few times w/ ur fist and see if u can hear a rattle? If you can, its your y-pipe.

Also check out you cat...not sure if u have an after market or what, but it might just be that it wasnt welded on right (that is my problem right now)

Check heat shields over the cat to see if they are loose.

Check clearence on exhaust system, see if its hitting anything under the car, cross bar, frame, axle, ect.

My y-pipe was shot and i had to get a new one. The stock y-pipe has 2 layers, smaller pipe inside the exterior larger pipe. The smaller interior pipe rusted out and formed a rattle. But my noise was more distinct when i fired up the car in the morning.

I have a new rattle now where i hear it PERFECTLY at 1400-1450 rpms. That is due to the inproper welding of the cat (i belive, i'll know in a few days) It sounds like somethings ratteling and sounds like some exhaust is escaping from the side cause i can hear little wistles.

Not sure, check out a few of those things i mentioned and keep us posted. I'm kinda curious myself [img]smile.gif[/img]

Good luck. [img]graemlins/thumbsup.gif[/img]

[knowledgefreak]

bad y pipe or welding rattle occurs at certain frequency / rpms if you hear ratteling all the time or in the muffler your cat is clogged and heated up and broke apart and is now coming to pieces in your exhaust pipes or will cause more clogging. Ses light will happen soon if it has not already.

[Wicked 3800 V6]

I have the almost exact same problem ... rattling when off the throttle but up in revs, particularly between 2000 and 2500. I'm checking on it in about a week, and will keep everyone updated.
-Rob

[chas]

i had a similar problem with my '99. there's a stupid heat shield on the i-pipe that's between the i-pipe and the gas-tank (or so i think). it looked like annodized steel. annodizing is just pointless on things like this. just ripped the damn thing off. my damned ford had this same problem too. the heat shields are on the exhaust pipes. hello?!?!? they vibrate. the dodge has the heat sheilds affixed to the body. much better. when they come loose they dont rattle and buzz.

and no i am not concerned about going up in a big ball of flame.

[GregWS6'01]

I have the same problem with my heat shield....I think he's got a different situation though.......

[GregWS6'01]

I have the same problem with my heat shield....I think he's got a different situation though.......

[Bluefury99V6]

Well I got underneath the car today and couldn't narrow down the vibrating noise to the CAT or to the shield that is around it. However I did see that my rear end is leaking bad! :eek: It's leaking right where the drive shaft connects to the rear end. I'm over my warranty by a month and have 35K miles. I gave Pontiac a call and they said to take it into the dealership and have them look at it. After that I'm supposed to give Pontiac a call and they'll tell me if they will repair it under warranty or send me up sh#t creek. This is my second Firebird. My first was a 91 and 3 years after that (12 months of payments left) warranty went out that car went down hill. Almost like they build them like that. That one started out with a starter problem and they fixed that under warranty. Then it just went to hell. I lovingly refer to my 91 red WS-6 formula as Satan. I hope this isn't a repeat on this car. This car is my fourth Pontiac and fifth GM car. If they do me wrong and this car goes to sh#t too, I may turn to the dark side (something other than GM).