strange sound when i begin to accellerate?

[davembfan]

I have a auto sedan yaris, when i first turn the car on and put it into drive, as soon as i hit the gas I hear something that sounds like pressureized air releasing. almost sounds like a blowoff valve but not as pronounced. I noticed someone else heard it from outside my car and looked at it like wtf was that. just wonderring if its normal and if so what is it? never had any cars that did this...
thanks!