I think the problem is primarily with the belt and brush roll. You can test my theory by disconnecting the belt and listening to the sound of the motor. See if it is consistent or if it powering up and down.
From your photo I can tell that the belt you have installed is too wide for that nozzle!!! Belts definitely matter and are not created equal.
The proper belt for that unit is a Hoover 38528-011-- see link below. The width of the 38528-011 belt is 13 Millimeters.
For your brush roll, I would recommend removing it from the housing. Unscrew both end caps. Clean and remove any hair and dirt and wipe the underside of the end cap. Directly below the end caps are bearings on each side. You can pry off the white plastic cap. Clean the white cap, Gently scrape as much gunk out of the bearing as possible. You can hit it with some WD-40 or similar spray if really bad to loosen it. Then repack the bearing with blue grease. I use a flat head screw driver to manually pack the bearing and turn the bearing on occasion to get the various balls of the bearings distributing the grease throughout.
Directly below the bearing are metal washers. The washers should be completely clean and grease free. Lastly I wipe that entire area of the where the bearings and washers are housed. Get rid of any dirt, grease, build up.
Reassembly of the brush roll is then crucial: Washer in first. The raised end of the bearing will face up toward the bottom of the bearing which will sit on top of it. The down facing sides of the bearings should face down towards the inside of the brush roll. Bearings go on top of the washer. Ensure that white or black plastic top is firmly secured below the metal top. Bearings should rotate smoothly with ease. The plastic part should be visible on top! End caps get screwed on.
Then to test brush roll:
Stand the brush roll on its end on a newspaper or cloth but not a dense carpet. Support the brush roll by placing fingers on end cap only. Use free hand to quickly spin the brush roll middle. It should rotate with ease, very smoothly and not get stuck at all anywhere!
Reassemble your nozzle and see how it sounds then.
Another issue you will face is that the bristles on your brush roll are getting quite worn though too. Hoover has discontinued the replaceable strips and the brush roll itself. There are some vac shops here and there that have found replacements or you can track eBay.
Any Hoover power nozzle with a 2 prong flat plug and wand set with the raised button set up like yours has will work on that unit!
Best of luck.
http://hoover.com/search/?q=38528-011&x=0&y=0