Check all the easy things first. The clamps on the Y pipe, throttle body, just revise them all. Check the torque on the long bolts that hold the manifolds on. If some are loose you found your problem. I'd check them to 15 nm so you don't break them loose if they're still tight enough. (spec is 20 nm) They have a thread locker on them and if you break them loose you'll need to remove them and reinstall with loctite blue or similar. Also, the torx head is made of soft metal (lot's of these on this engine) and you'll round out the bolts head before over tightening. The stock gaskets are fine for stock boost but when the boost gets around 18 or more PSI they filter boost ...... at least that's my experience. Post up what you discover. BTW, great write up on the motor mount mod. You'll need tips if you use the cork gaskets. I messed up several times before I figured out what I was doing wrong.

Les