Ok, I just got my 98 Firebird Formula a week and a half ago. When I first got it the muffler was chopped off about 6 inches after the Y-Pipe. That sound was a little obnoxious and got me more attention from the cops than I wanted. So upon the advice of you fine people I purchased a Hooker catback system. I got it all installed ok (with the exception of a missing hangar) But now when I start the car up (from a cold start) the muffler like hums or vibrates it only seems to last a few minutes then goes away. However I also noticed tonight driving home in traffic that as the RPMs drop below about 1000 the humming returns. It is noticably loud and pretty annoying. Any ideas what is causing this ? maybe the new muffler needs to be broken in ? I have no idea...

 

I heard that thats normal when the car is cold from a guy with a fartcan , TTT for a knowledgeable answer .

 

Is your cat back system still bolted on or was it welded? If it is still bolted on, go back and tighten all the bolts on all the clamps. If that doesn't fix it, check around the hanger and above the muffler for any metal to metal contact points. If your hanger is too short, your muffler may be rattling against the heat shield above the muffler.

 

It is still bolted on and I had to substitute a 3" clamp where there should be a 2 3/4" That will be replaced on saturday and the left hangar is missing (we made one out of some plumbers tape) As for the heat shield why would it only be at lower RPMs and when starting up ? I will tighten everything up on saturday morning and see if that does it, I am going to try to get a friend to weld it but I'm not sure if I want him under my car welding **** together (hes my best friend but hes a little insane!) We'll see what happens Saturday, other than that any ideas tho guys?

 

It could only be rattling when the metal is cold until it expands and gets a direct, constant contact. I would definitely get a smaller clamp also. Word of advise, if you plan on doing a lot of work to your car, especially suspension, etc., then leave it bolt on. If you get it fixed Saturday, let us know what it was.

 

My Hooker catback does that as well. Someone told me it was vibrating up against the heatshield near the gas tank. I haven't actually gone under the car to see if thats what it is though.

 

That's what I was suggesting to look at. I know people that have had that problem before. It could be that your hanger is too short. Extending the hanger about 1" could help that. Just need a welder to do it.