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Discussion Starter #1 (Edited)
Hello, I have a 2009 Cobalt LT with 63k miles.

Coming home yesterday I noticed a noise coming from the right front side of my Cobalt whenever I applied the throttle. It sounds like an extremely loud valve rattle, high pitch in frequency. I'm used to hearing the valve train running, it's done that ever since I got the vehicle and this sound is in time with it. Where the valve train is a steady clicking, this is more of a loud buzzing sound (like vibrating an aluminium can).

My quandry is this: The sound was only occurring when I was in gear. I tested it in park, and popped it into neutral while driving on surface streets and took it up to 3500-4000 RPM, the sound was not present. However, whenever I accelerated any amount above normal engine idle (about 1200 surface streets, 1500-2500 depending on highway speed) the sound started and lasted through the entire RPM band. When I stopped applying throttle the sound cut out completely.

I got into the garage and put it up on stands to have a look at it today. The heat shield material (some kind of fabric mat) between the top of the exhaust system and the engine bay has come away from the body toward the ground and has signs of minor damage (floppy, some minor tearing). I tried wiggling exhaust pipes and checking for loose bolts, they're all good. The aluminium heat shield along the exhaust pipes and over the catalytic converter is solid. I fired the engine and crawled back under it to see if I could feel/hear anything vibrating that shouldn't, it all seems to be fine.

What the heck is causing this? It's not speed based, it's happening anywhere from 5mph to 80mph. It doesn't seem like a knock or bad rod bearing because it's only happening when in gear. Should I be checking my engine mounts? Could the transmission be getting ready to let go?

Any help would be appreciated! Thank you.

EDIT: Transmission is automatic, no warning lights have come on, and I don't think the engine has set a DTC. I'll have it in the school shop monday and I'll grab a scan tool to see if it's detecting anything.
 

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Bussing sound from directly behind the glove box area is almost always a flex pipe leaking. Mine has done this for 50k km. You can get just the flex pipe piece or you can buy a downpipe, its up to you
 

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Thank you Money, we got the car up on a lift today and isolated it to the flex pipe. Now the trick is finding out if it's the gasket our if the flex pipe itself has a hole. Fortunately I have access to welding equipment if that's the case.
 
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