Chattering when car is off, dead cold

Snowed in in the misty woods today, so decided to take my FIRST look under the hood of my '08 with ~400 miles on it.

After checking to see that the right amount of oil had indeed been filled, I replaced the dipstick and as I straightening up, a quick chattering noise -- almost like a rattlesnake sounded just ahead of me....If I had to guess I would guess it may have been coming from that black box that says "relay" on it.

Noise lasted about 1/8 to 1/2 second or so

The car is dead cold. Hasn't been driven for some 24 hours and the ambient air is about 23 degrees....so I KNOW it wasn't a rattlesnake -- in this weather it would have been a snake-sicle. Gee, maybe it was just so cold that the car's teeth were chattering

Any thoughts? Is this common? no warning lights when I moved it some 40 yards yesterday morning...

 

I really can't say how long it lasted....as mentioned....I guesstimated about a 1/2-second....but I was so startled that it took me a second or so...so outside length maybe 2 seconds....maybe as little as 1/8 second, but probably longer...

Interesting about the brakes. I know about Hydralic assist, but electric..

 

Man, what a car....a far cry back from the days when I used the belt from my Levis to replace a broken fan belt on my '62 Ford Custom to enable me to limp into town.

I suppose that next you folks will tell me that the Prius measures my blood oxygen level as I clutch the steering wheel and feeds or restricts fresh air intake into the cabin, based on the results..