Light intermittent grinding noise

My zr550 started making a new sound recently. Can't say I like it.  It's kind of a very light sporadic grinding/rattling noise that seems to be coming from the middle or the rear of the vehicle.  I don't hear it when I rotate the wheels with the bike on a stand, but I do hear it coasting down a hill in neutral.  Maybe my weight on the bike is the difference?  I thought it might be the chain, but I recently lubed and adjusted it, and I didn't notice any kinks or binding when it was on the stand. Maybe a rear wheel bearing? Any other ideas? 

Thanks all!
Mark

here’s a recording of the noise, coasting down a hill in neutral…

drive.google.com/file/d/1LGpaC7FR04wHRO8...sg/view?usp=drivesdk

Check the bearing of the output shaft behind the front sproket.

ah, you may be right. That would be a big job to replace, correct? 

Yes, you have to split the engine case and take off the lower half to replace the bearing.

How can I diagnose an output shaft bearing failure definitively? 
Think the bike is safe to ride even if the bearing is going bad or do I risk a catastrophic failure? 

Good thing I have a spare engine just in case...

Remove the chain and front sprocket. Rotate, push, pull, move the shaft in all three directions.
If you can feel/see/hear any kind of play, wooble, grinding, clicking your bearing is shot.

When an output shaft bearing fails, it can cause catastrophic damage to the engine cases, and could lead to serious injury or worse. 
Imagine the left side of the engine exploding while you are humming down a highway. Maybe it does not explode, but merely cracks badly, suddenly locking up the transmission and blowing a liter of boiling hot oil on your left foot and the now locked up rear tire. Maybe you will be lucky, and it will fail as you are pulling away from a traffic light, causing you to come to an abrupt halt while the inattentive driver behind you wonders why there would be such a large speed bump after a light. Maybe it will happen in a less painful way, but maybe not. How lucky do you feel?

Well, good news is it looks like the output shaft bearing is fine, no play at all. That's a relief.  Bad news is I still don't know what the noise is and if something is going to grenade while I'm riding. I don't know what else it could be other than the rear wheel bearings.  I haven't take the rear wheel off but on the stand there is a very slight bit of play in it left to right.  Is that normal or should it be rock solid? I think that play has always been there. 

 

I'm not familiar with the Zephyr 550 but if its chain drive like mine it should be easy to remove the rear wheel, remove the sprocket carrier, remove the dust cover over the bearing (unless its sealed) and have a look. I had a couple mm side to side movement in rear  wheel and then one day what sounded like a gunshot from the rear. The next day another gunshot and a wobble as I slowed at the 2nd light from home. When i took the cover off the bearing it was ugly in there and few of the balls had disintegrated. I have zero side to side rear wheel movement since replacing rear bearings several years ago. I have 3 bearings in the rear, 1 in the sprocket carrier and 2 in the wheel hub. Replacements were AllBalls
Hope its as simple as that

You might want to drop your oil pan and look for any metal.  If you have a magnetic plug you could start by checking it for metal.  If it's clean then your internals are probably OK and your problem would be elsewhere.  

Just to close the loop here, good news, it was just bad rear bearings.  Lesson learned.  I've always had just a tiny bit of play in the rear wheel.  That's not normal. Rear wheel should be rock solid. Any play is an indication bearings might be shot.