Popping sound @ Idle when engine cold, any ideas??

[429mustang]

My car makes a weird noise kind of like what your talking about, it sounds kind of like blowing air through a straw in water (it's not real loud), it does it when the motor's cold and sound like it's coming from the rear of the engine by the firewall. I looked under the hood and it isn't coming from my smog pump, although your noise could be something different from mine. If I find out where it's coming from I'll let you know.