Coolant Pump Farts

I don't know how else to describe it, but my '08 with 13k miles occasionally makes a 2-3 second "farting" noise at startup. It's not the normal humming sound I had in my '05. It seems to be coming from the coolant transfer pump area under the center armrest.

I've seen others describe a "squeal" or other high-pitched sound, but this one is pretty low-pitched. Yes, I have made sure it wasn't me (even though each time I hear it I happen to be in the car at the time).

I've heard there's a TSB for a "squeak" sound during cold weather. It's been fairly hot here recently, and it did not make the noise last winter (I've had it since December). Not sure if the TSB applies.

I haven't taken it in yet -- I'm afraid they won't be able to reproduce it and they'll think I am crazy.

Anyone else had an issue with Prius flatulence?