I just did some random Googling on this out of curiosity. I found one similar reference the guy didn't get an answer to.
When pushing and pulling vacuum across carpets there is a strange noise, sometimes an electrical smell a rattle and vibration on the roller housing. (I've been naughty and observed and felt these vibrations with the top cover removed). Could it be related to the clutch mechanism built into the brush spindle?
Well, you have changed the clutch mechanism, so it isnt that.
I am guessing you have ran it with the belts removed one by one and discounted the brushroll?
Surely that leaves only a bent motor spindle or motor bearing issue?
Yup, if you watch part 1 and 2 of the repair video's (If you can find them among the other videos) I have taken each belt off, replaced the brush roll bearings, sensor and inspected the belts and motor... The motor runs great on its own, and feeling the spindle/bearing with your hand there is no play or flat spots...
I'm going to replace the belts, if that doesn't fix it it pretty much has to be a motor issue, somewhere... I recon