I got my car back about a month ago from the shop, got a s/c installed. It's been running fine but the other day I swapped out the factory plenum for another factory plenum that had been polished some. Now when I get to about 2500 rpm the car makes a quick noise that sounds like air pressure being released from the exhaust along with a slight hesitation maybe every 10-15 sec or so. I started paying extra attention to the boost gauge that was installed and realized I dont really know what I should be looking for. I know what boost does but mine is under load the needle goes from maybe 20 or 15 to around 0-5. However, the needle starts heading back to 10 or 15 before I let off of the throttle. Is that normal? I didnt watch it THAT close before, so i dont remember if that was normal or not. Is it also possible that the new plenum I put in is letting air escape and that explains the noise i hear?