Pump like sound from the rear in the middle of the night?

I think this has been mentioned before but can't find it. Late last night when I was changing the transaxle fluid I heard this pump like sound (somewhat muted) coming on for a long time, maybe 5 to 10 minutes. It seemed to come from the rear of the car. The car was off at the time. The ECT spoofer indicator lights also came on (and went off when the pump sound went off). Anyone remember what it was? Maybe the brake capacitors? Thanks!