I recently had the Magnaflow 2.5 in catback installed (part #16815) and ever since I've noticed a whistling sound or the sound of air escaping or not getting somewhere. So i had it check for leaks and they couldn't find any so i took the mechanic for a ride and he said thats the back pressure from the catalytic converter. I've also noticed a drop by about 5 mpg's which I'm sure is not normal. Has anybody else noticed this? Or could tell me what it is.

 

Also the sound only occurs while driving and stepping on the throttle, noise starts at around 3,000rpm and gets louder from there. Does not make the sound while standing and just revving the engine

 

Has it been going on for a while? The 5mpg might just be the heavy foot from wanting to hear the exhaust :P How many tanks have you gone through after installing the exhaust? Was the exhaust the only thing that was added/changed before it started making the whistling noise?

It could be a couple things, if you can you should check all the connections on the exhaust, make sure they're nice and snug.

Edit: took out the bit about the header, lol, just noticed this was from a 2nd gen car...

 

It started making the noise the day it got installed I've gone maybe through one tank of gas if that. I had the connections checked and their snug, it's the only modification to the car so far I'm waiting for a CAI to come in. It also doesn't make the noise while the car is cold, usually by the time I hear the pop and burble of the exhaust is when it starts whining.

 

I would check the clamp/gasket from the dp to the cat back. Also check the pipe fittings to make sure they are fitted correctly. Mine did the same thing and I found the clamp wasn't tighten enough.

 

I saw somebody say in another thread that you need to use a 1/2inch drive to tighten all the connections because a 1/4inch won't give you the necessary torque. Not sure if that applies to you but it's worth a shot.

 

Il check the clamps this weekend when I install the CAI and hot side tube

 

I have the same exhaust and I started noticing something very similar once I had a restonator put back on. It was clamped on, so I'd have to say that my first guess would be to tighten the clamps. I haven't been able to check my clamps as I don't have a garage and it's winter... ugh.

 

My CAI isn't coming in for another two weeks :/ so il just have to check then I don't feel like splitting up the work