08 cobalt 2.4 with 85k miles and I'm trying to solve this engine noise. This might be long but I'm out of ideas without replacing parts and spending money. I'm leaning towards timing chain slack or flex pipe/cat. It's hard to hear it in the driveway or on the open road, but cruising through town or beside buildings it's easily heard. It's hard to explain, it's kind of a static like noise coming from the engine area and rpm dependent and sometimes accompanied by a faint high pitched rattle. Sitting still at idle it's smooth and quiet until I take off and the noise gets louder with rpm, louder than the exhaust unless I get on it. Doesn't matter if sitting still revving, moving, gear/neutral, clutch in or out all have no change on the noise. Car still runs and drives with no issues. There's also a whistling noise that comes and goes around 3k rpm, I'm not sure if it's related but the whistling happens mostly when the engine's cold.
I've had someone rev the car while under the hood and I'm not hearing it the same as when driving, making it hard to point out. Couldn't feel any exhaust leaks either. The only noise I could locate was a slight tapping/rattling noise coming from the bottom engine area around 3k rpm, and I could only hear it when I was laying under the car, didn't sound like rod knock or anything serious. After that I decided to check the timing chain.
I removed the valve cover and the chain looked and felt perfectly tight. Then I rotated the exhaust cam forward a few degrees, actually I'm not sure if the cam moved or just the chain, but it pulled the slack to the top. Here's a video of the chain https://youtu.be/kbsjavBnBWk
Does anyone know if this is an acceptable amount of slack or could this cause my noise? No guides were broke or damaged looking, so I'm guessing tensioner may need replaced.
This has a two cat stock downpipe. Cats don't rattle when shaken or smacked. Using an ir thermometer they seem to be around 500 degrees with about a 70 degree difference front to back. I'm not feeling any leaks around the flex but I know under a load such as driving would create more back pressure and possibly leaking only under that condition.
I've also removed the serpentine belt and took it for a short drive, noise was still there. Injectors also sound normal. I'm hoping someone has experienced this noise before and might have a solution. No other problems, the power does seem a little flat around 4k sometimes, but that may be normal. At this point it's just an annoying noise.