Really interesting noise - anyone heard it before? (w/ video)

I heard a disturbing noise from my Prius earlier this week. I have done 300 km since, and the noise has not recurred.
I didn't bother checking in the parking lot, as I was convinced that something was toast, and it would be easier to confirm at home.

Here it is (from dashcam):

It sounds louder in person and seemed to be just as audible outside the car.
Heater switched off by me midway, drove normally, no check engine light, heard ABS do self test (sounded normal.

Has anyone heard this noise on theirs, and was it possible to confirm the source?

 

I switched off the HVAC midway to see if it had any effect. As far as I could tell, it did not.

Gen4 comes tomorrow. My wife will be driving this one, so I better get it sorted!

 

Thanks for the replies. Coolant level is good. (in rad, in overflow and also in the inverter circuit.)

I have heard many a bad water pump in my days, and (fortunately/unfortunately?) this sounds like none of them.

 

When you say 3-way valve, are you referring to the water pump for the heater? This vehicle is a is Gen 1, so no coolant tank on it (unless I am misunderstanding your idea).

I re-listened to the video, and I think the noise you are referring to is coming from the cassette player. I was given some audiobooks to listen to on my commute, and they are on cassette. Cassette player volume is turned down in the video.

 

Any more takers on this?

Strangely, it hasn't done this again. Maybe it is aware of the '16 that it is now sharing the driveway with, and has decided to be good.

 

I think this might be it: Grinding noise for 20-30s at cold startup... | PriusChat

Crappers.

 

Follow-up for any that are interested.
I hope not to jinx myself, but the problem has not recurred since the original video was taken.