Hey guys, just got a 2010 Cobalt ss/tc a few months ago. My brother got it new a few years ago, he had a bit of an accident and left the car out of gear, without the break on and had it roll into a tree. I took the payments from him, replaced the oil pan and intercooler among a few other things, and the car was running well.... for the time being.
For a few weeks now the car will boost normally (around 12-14psi) with the normal very feint turbo spooling noise, and sometimes it will boost to maybe 7-8psi with a much LOUDER spooling noise, the same type of noise just louder.
I checked for a boost leak, both visually inspecting and with an air compressor. I've checked all over this site and google and cant come up with an answer, any help will be appreciated.
And the only thing that seems to help that I saw somewhere on another thread was disconnecting the battery. After doing so it will (somewhat) act normal for a while and then go back to flipping from normal boost to low boost again.

 

Check the bpv diaphragm, check all sensors to make sure they are in tight. The check valve in the boost box(small black box under intake manifold) could be going bad.

 

Thanks for the response man, I'll check everything out tomorrow when I get out of work and see what I can figure out.