clanking sound from engine

[chunts]

I've got an '01 gs500 I've owned for about 6 months. 23k miles on it, but generally the thing is in surprisingly good shape. A couple of days ago I started noticing a troubling sound coming from the motor sometimes. It's pretty subtle and comes and goes, so it's hard to really pin down. The best way I can describe it is like a periodic clank/rattle, like if you had a wrench going around in your clothes dryer or something, but much much softer. More than that though, I can actually *feel* the knocking through the handlebars and the frame. Again, it's very slight, but definitely there. Here's what I can pin down:

* Sound comes and goes. sometimes its really defined, others I can't hear/feel it at all. can change moment to moment.
* The rhythm of it is in perfect sync with the motor, but not on every beat. It seems to really hit every 4th or 8th revolution, or some other interval.
* Happens at idle in or out of gear, clutch in or in Neutral.
* Can't tell if it increases with engine speed cause the motor drowns it out at higher rev, but I can't really feel the clank then either.
* Checked oil on the centerstand and it's fine. Barely over the F line.
* This morning it was doing it a little more and it *seemed* like it was coming from the lower left side of the engine, or at least it was easier to hear there.
* It's not super loud yet but I can tell it's getting a little bit worse each day.
* Bike otherwise seems to run fine

No mods on the bike really, I bumped up my pilot jets one size but otherwise the engine is stock as far as I know. I'm pretty mechanically inclined, but also pretty new to motorcycles in general so I don't have much experience tracing down what this might be. I searched a bit here and the most likely thing sounds like the counterbalance bearing, but that's just a guess. Coincidentally, the side cover over the countershaft bolt(?) seems to have departed from the bike in the last couple weeks. Maybe something is rattling around in there? Anything else I can check for?

[adidasguy]

Seems to be the same problem Junior had: counter balance bearing.
He ran perfect: power, idle, everything. Except it clanked like crazy. Then one day the bearing slipped around and WOW! What a thrashing machine. Still engine had power, shifted fine and everything else was OK except for  the noise.

Here is the thread on Junior. You might find some useful information there and photos & video where I identified the bad bearing. Drain oil and take off right side cover to check it.

Links:
http://gstwins.com/gsboard/index.php?topic=60331.msg696908#msg696908

http://www.youtube.com/watch?v=Atq4nKfh7kg