If you were in the car closest to the front power unit, you could be hearing anything from traction motor 'whine' to transformer and/or motor cooling fan 'whine', or even regenerative braking motor sounds (electrically powered locomotives 'dymamic brakes' generate electricity back INTO the wire!)
I'm also inclined to say 'singing trolley wire' sounds are being heard. Back in the days of interurbans traveling 40+ mph in rural areas, one could easily hear the 'singing' of the trolley wire from the slider shoe on the trolley pole pressed against the wire as the car moves along. You'll still hear it at any operating trolley museum these days. Think of a violin string being played by a bow. So it's quite possible the pantograph is causing the catenary to 'sing' as well.
Or....
In the first years of Superliner equipment ('79-80 delivered), I had occasion to be in the handicap bedroom as that was the last roomette available. My room was filled the entire trip from Oakland to Chicago with a 'hollow sounding' whine that would change pitch with speed. The rear wall of the handicap room (and family room, too) was nothing but a single sheet of metal between me and the wheels. 3 years ago, I had to make reservation changes a week before travel and ended up in the family room. It was obvious that Amtrak and/or Pullman added sound insulating material on those walls.
For what it's worth, the same hollow 'whine' sounds were clearly heard in the 'Heritage fleet' equipment if you were in roomettes 7-10 and held the toilet pedal down and watched the ties whiz by underneath. The public toilet at the other end of a 10-6 was directly above the trucks, too.
