Drive shaft noise

[furok]

I have a 2002 1500 Drifter. It has 52000 miles on it. I recently changed the oil both in the case (with a filter replacement as well) and the drive shaft. After much reading online I decided to try synthetic in both. They both have only had conventional oil in them before. It is the first time I have replaced the drive shaft oil since I have owned the bike. I don’t know when it was replaced before I owned it. So, it could easily have been 30000 miles (or never) since the drive shaft oil was replaced. I replace the case oil every 3000 miles.
I used 10-40 in the case and 75-90 gear oil in the drive shaft.

Now for the problem and questions for you all. A few days later I took the bike on a long ride. After I stopped to fill the bike up with gas I pulled out, sped up to 55mph and then had to slow down for a light. When I disengaged the clutch (I pulled the lever in) I heard a sound that I couldn’t identify. I would describe it as a low pitch, but a lot like the sound you would get by sticking a baseball card in your tire spokes as a kid. When the clutch was engaged (released) the sound went away. I rode it a little farther and the same thing happened whenever the clutch was disengaged, and I was moving at speeds above 45mph.
I decided to get towed home.
I have also been able to get the bike to make the noise when in every gear except 1st. I can get it to do it in neutral but only if I am going over 45mph. It is also somewhat random. It doesn’t do it every time in every gear. It always does it in 4 and 5 once the bike is hot. The bike must be moving to make the noise. I cannot reproduce the noise if the bike is at rest no matter how hot the engine is.
Over the next 2 weeks here is what I have done:
1. Replaced the case oil with conventional 10-40 – NO CHANGE.
2. Replaced the drive shaft oil with 75-140 (still synthetic) – NO CHANGE.
3. Checked all the bolts around the engine, exhaust, etc. can’t find any loose bolts to cause a rattle of any kind.
4. Raised the back tire off the ground to check for any play – NO PLAY, nice and firm.
5. Raised back tire, put it in neutral, and spun the tire – no sound at all (other than brake pads rubbing on disk).

And the last time I rode the bike I was able to hear the sound for a fraction of a second when I released the throttle but not disengaging the clutch. I hadn’t noticed that before, but I also don’t know that I was specifically listening for it before so it may have been doing it.

I am pretty sure it is drive shaft related and not clutch related. I am also pretty sure it isn’t the front bevel gear or u-joint since there is no sound when I spin the tire in neutral and no wobble at all while riding or when the tire is up in the air.

Does anyone have any ideas? Could it be that the rear bevel gears at the rear tire have gotten slightly out of alignment? Is that a hard fix? Engine work isn’t a problem for me (I have done the top end on this bike engine before).

Thank you all!