Mystery noise behind passenger side dash - help me identify it?

I've put a recording on Soundcloud:


The noise is best heard at 1:00 onwards, after everything else has stopped. I left everything else in the recording so you'd have an idea as to the volume level relative to the engine etc.

Any ideas? It sounds like some sort of solenoid repeatedly trying to do something, or a broken fan or pump.

We already had the dealership attempt to fix the problem once, so I'm hoping someone will have encountered the same noise and recognize it.

Thanks!

 

This is a 10 year old Prius. The noise is audible when the car is completely stationary but in "engine on" mode. The noise starts and stops seemingly at random.

And yeah, I joined when I was looking for a Prius 10 years ago, but the intervening period has been pretty uneventful. The car has just run and run. Probably time to buy another one next year, but as I say I already paid to get this noise fixed, so I'd like to solve the mystery.

 

OK, some more info.

The noise seems to be heat related — when the car's cold in chilly weather, no noise. As the car heats up, it gets noisier. When the engine's been running a while and you stop, the rattle is audible until the car cools down.

Also, we tried stopping and opening the hood to locate it more precisely, and it seems that although inside it's most audible on the passenger side, when you lift the hood the noise is from somewhere to the right of the Hybrid Synergy Drive box. Unfortunately, I've no idea what's down there?