Does anyone know what the unusual sound is on a 717 that happens approximately one minute before the engines start? Perhaps it is a pump -- it sounds a little like a pump, or even a brief, muffled electric drill. It goes "wrrrt!" for about a second, then stops for 2 - 4 seconds, and then does it again a few more times. The cycle repeats maybe 5 times in total.
I mostly notice the sound towards the rear of the aircraft, right around push-back time.