Here's what we're working with.... '00 Trans Am 6spd car. 61,200 miles. Only modifications: Functional firehawk hood with intake and SLP loudmouth.

Here's what it's doing:

At idle there is a definitive "squeak" that you can hear directly underneath the car, as though it is coming from the exhaust. When the car is turned off you hear a "psssssh", as though there is pressurized air being released in to the atmosphere OR air escaping through a small hole.

After searching on LS1tech and numerous other sites, I had a few ideas of what to look for... At first I was thinking a slight exhaust leak issue, so I tightened all the clamps on the catback. It quieted down, but not gone 100%... Then I eliminated "fuel pump noise" as another poster noted by topping off the gas tank to full.. Still not gone...

I've shown it to a few people and everyone is at odds: Valve train needs help. Or emissions equipment (IE: Air Pump, Check Valve, or Cats starting to clog up.)

Now, the car has run the 1/4 mile and traps at 106mph. It was making the noise then as well, so it hasn't hurt it's power, as I think 106mph with just an air intake and catback is fairly decent.

SspdDmon and I were looking at it and we noticed that when the car is turned off, air seems to rush back out of the engine THROUGH the intake... He is leaning towards valve train issues and suggesting a compression check - which I will be doing... But I would also like to know what else I should be eyeballing... How do I check to see if an AIR pump check valve is good or not?? I tried removing the AIR pump fuse to see if this would eliminate the problem and it did not....

Where do I go from here? The only reason I haven't checked the compression yet is I do not personally own a tester......

 

I have a LM on my Camaro and I also had the squeak. For me it happened when I let off the gas. I fixed it by adjusting the the height of the catback up just a bit.

 

That may be it - because I can hear it when I decelerate at low speeds (lol, the LM overpowers it at any sort of acceleration)... That could explain it. I bought the car with the exhaust already on and didn't have much time to drive it before putting it in storage. When I checked the exhaust it was so loose that the over-axle pipe was SLOPPY!!! Maybe it's way out of whack/adjustment??? Once in a while I swear I hear it vibrate/touch the underside of the car......

 

Anyone else? I haven't had much time to tear in to it - but I did have a second to break out the stethoscope again and listen to the motor. The drivers side check valve does appear to have some high pitched noise coming from it when the motor is warm......