What's this sound?

Been going on for a couple of weeks now--what with all that's going on I haven't been able to get it looked at, plus it doesn't happen all the time so I was waiting until I could get a recording of it.

It happens most when the car has been sitting for a while. I don't mean cooled down: I can park for a few minutes and then hit the gas, and there it is. The car doesn't have to be moving; I think, but I'm not sure, that it will happen if the ICE is running at idle, without the pedal being pressed. It's most noticeable at lower speeds (RPMs?), but I've heard it between 30-40, when I've used the gas pedal to maintain my speed.

I think it's emanating from somewhere lower to the ground, but I haven't had someone around to press the pedal while I've had the hood open, so the best I could do was crouch down with the door open and press the pedal with my hands.

Here's an audio file.

Any help would be welcome.

 

Thanks for the replies. Got to get it looked at, but at least the engine isn't about to seize.

 

Turns out, it was the intake manifold--the manifold itself, not the gasket. Car's at a local dealer now while they order the part. The rep said it took some doing to pin down the noise's source.

I didn't make the connection earlier, but my mpg's have been down... I suppose this could explain that. But neither my OBD connector nor the dealer's testing yielded any error codes.

 

Air was getting sucked in, so I guess something was cracked, and that vibrating metal (where air was coming in upon acceleration) caused the sound.