Gears gone wild?

Background: VCR, General Electric, Model 4272. Never had any problem before. Observations: There is a series of gears between the 2 reels. None appear to be chipped, worn, or broken. There is no idler wheel or tire on this model. The belt is tight and not slipping. With machine unplugged and tape out, if I manually turn the pulley, even with pressure on the reels from my other finger, the gears mesh strongly, do not slip, and the belt does not slip. When I plug in power and put in a tape, I hear gears whining and the reels do not behave like gears in line...but only with the tape in....??? I don't know why.