Weird noise from rear of the car.
 

Mostly happens when car is cold, not warmed up.
It almost sounds like the low hum of a coffee grinder.
At first I thought maybe the auto lock mechanism on the hatch was running even though the hatch was closed, ruled out that though.

I think it may have something to do with the suspension.. (So what else is new ) The sound seems to change quality when I lower the suspension. It sometimes even goes away. But when the suspension re-adjusts, it comes back.

Oh yeah, I only hear it inside the car, not outside.

Any thoughts...

Probably stating the obvious here, but is it the air compressor running, or in that general vicinity the noise is coming from?

Also bear in mind that your ABS gear is sat just under the spare wheel in that black dome, have a listen there.