Throwing In the Towel...
Well, after looking inside the PN and considering all the responses here, I think it's just meant to be this way.
First, the motor mount has no adjustment slots, and the belt is not loose.
Second, the sound occurs only on the "pull" stroke. It does not occur when you push the PN forward, only when you pull the PN towards you. That would seem to argue that the noise has something to do with the brushroll spinning in a forward direction while the unit is being pulled backwards. In other words, something you can't do very much about.
Third, Mr. Charles Richard Lester hath spake that the damned things are noisier than the canisters. Considering Charlie has been fooling around with Electroluxes longer than most of us have been collecting, I figure that's the final word if ever there was one.
