I just noticed that recently, when I rev my engine at idle I get a pshhhh sound when I let off the gas that's similar to that of a turbo (although not as loud). Anyone know what that is?

 

Air control Valve releasing air pump air to atmosphere (via silencer assembly that you may have removed)? Look below the intake area on the passenger side of the engine, if you don't know where the Air Control Valve is.

 

It's your vtek.