This has got me stumped. Usually, when a rev-related sound comes from the engine bay of my car, it's a vaccuum hose or something. But they are all on and clamped.
But, when I sit there and blip the throttle, maybe to about 3 or so, then let off, I get this "hisssssss", then a "pop-pop", from up front. rev it... "hisssssss pop-pop".

anyone else heard of this before? thanks

 

If it only happens when the car is warming up or when the weather is cold, it might not be a big problem.

 

If you hear an odd honking sound like you ran over a duck it's you coaster shutter valve.

 

Its been described by other noises as well. I bet it is the shutter. It only works on deceleration.

http://www.mazdatrix.com/faq/shutter.htm

 

Yup, sounds like it could be the shutter valve. It is located on the drivers side of the intake manifold closest to the firewall. There are two rubber vacuum lines that come off of it and attach to the bottom of the air filter box. Block the large hose, it's about 1/2" diam, and that will disable the valve. Trust me.... ya don't need that POS it just causes trouble.