So, when I open the engine oil cap while the car is running I get this pvvvvfff noise, the motor doesn't change as far as hesitation or anything, just feels like air coming out. Is that normal?

 

Air should go in as crankcase is maintained below atmospheric to insure crankcase gas is recycled through the PCV and into the engine. No harm in removing oil filler cap w/ engine running but you may see a little outgasing due to loss of vacuum.

good luck

 

Why would you open it when it is running? You throwing in an additive?