Originally Posted by shapeshifter309
It even could be just one of the car's many hidden gadgets working, sort of like the blower (or whatever it is) that comes on when you first start the car up, especially on cold days. That supposedly has something to do with the catalytic converter and maintaining emissions right after start up.
That's the pump that injects extra air into the exhaust - the extra oxygen is supposed to help the cats heat up faster so they can do their job. (I read this online somewhere when I was reading up on a code that someone's car here threw.)
I don't think the noise is anything to worry about. Both of my roadsters have made odd sounds when I'm stopped but with the engine running. And like you said, it seems like the noise is common, but not associated iwth any failure or trouble.