Bubbleling noise from inside the engine.

I was doing a boost leak test to see if I had more boost leaks, Well there is a boost leak coming from the throtle body, I have that gasket. When you take off the oil fill cap, you can hear the bubbeling noise coming form inside,What could that be?

The compression should good compression across the board. It sounds like there is a boost leak inside of head or something.....:banghead: