new sound, not sure what to make of it.

I've owned this 1982 XJ750 since 1989.
in all the time I've put the 25,000 miles on this bike this is the first time I heard this kind of noise.
when downshifting, or engine breaking. I'm getting a gear noise or hum, only when under load, if I pull the clutch before letting off the throttle, I still hear it, but it's much louder if I put it under load. I'm just worried to drive it now, I can tell it's something with the drive shaft or rear end, because it doesn't change with different gears.
anybody have any idea's? is it something that makes the bike unsafe?
the rear brake looked pretty good when I had the wheel off last year, I was thinking about changing it, it was last replaced in 1997.
the rear diff oil was replaced with synthetic in 2010. I check it, is still looks good, and is still full.

 

Could be the final drive in the engine.
you would need to isolate the play in the final drive from the play in the rear end.
or maybe even the universal joint.
the best way would be to pull the swingarm

 

ok, here is a video of what it sounds like.
the bike is in neutral.
https://www.dropbox.com/s/2x5y7ug3rgherr3/VID_20150512_214302.mp4?dl=0

 

That's not good. Do you have a factory service maual? You will want to have one.

Things that it could be:

Worn wheel bearings (not likely based on the sound).

Worn final drive (very likely if the final drive was allowed to run low on oil or run dry)

Worn output gear (much less likely).

Worn U-Joint (the sound would be different, but it is worth mentioning)

Pull the wheel off and give the bearings a spin by hand. They should spin easily with no roughness or noise.

Then pull the left shock, and remove the final drive so it can be turned independently. It should spin smoothly and easily wothout any noise other than the sound of well lubricated bearings spinning.

The u-joint can be checked visualy by pulling the rubber boot back from the engine case.

After that the swingarm needs to be removed to check the output shaft.

 

It was never run low. I do have a shop manual

 

wow, I've been busy. but I hate not having my bike right now.
I got to it last night, and this is what I found. broken spot on the pinion gear. where the spring attaches as it goes into the shaft. I assume to keep pressure against the other gear?

 

here is a picture

 

Ya, that's not supposed to liik like that. What a highly unusual failure. That spring is important. The easy fix is to get a used final drive.

 

Found one on eBay. $32 including shipping

 

Guy prob lost money on that deal, good for you cheapest rear end job you will ever do!