I doubt that there is a correlation between the ampmeter spike and the rear noise. You could eliminate that it’s electrical if you take it out of gear at speed and then turn off the motor with the key so that nothing electrical is running. If the noise is still there then it’s not electrical.
My car has always made a slight shrill noise coming from the rear (different than another noise I recently got). Even after I replaced the rear fluid and got rid of the other noise and vibration I was getting. See
Jag IRS Squeal when turning
Please keep me updated and I'll do the same for you.
Okiesnake