front end "knock"
I have to agree with Voxmagna. My 99 K12RS started "knocking" recently and I had replaced the disc washers less than 9k ago. I had this issue on a previous bike.
Just reach down, grab the rotor and move it. You can feel the flex and sometimes even duplicate the knock. New washers time!
That said, on the way to the National in Wisconsin my bike got dizzy and fell over while stopped. When I picked it up and got ready to go I noticed a "knock" and some play in the front end. Being in the middle of nowhere and without my shop tools, lift, etc. I wound up having to trailer the bike back to Denver as I was leery of riding at highway speeds with "play" of unknown origin in the front end.
I suspected the "A" arm attached to the frame as it would move when grasped and flexed. Without a manual or parts book on the road I opted to wait until I got it home thinking that the pivot bolt that goes through the frame and holds the A frame had somehow broken or the frame was damaged.
When I got it home, I removed the fork tube assy. and could see the back of the A frame where it was attached to the frame. You need a good light and sharp eyes. I noticed an allen bolt that was loose. Before I removed it to put some loctite on it I checked the manual and discovered that there was a lock nut on the other side. Thank goodness I didn't pull it out or I would have wound up with a big headache trying to find the nut and getting it back on in a totally limited access environment.
I checked the torque specs. and re-tightened the bolt. Voila! All play was gone from the front end and the "knock" from the front end, not rotor related, had also disappeared.
I don't believe the "fall-over" was the total cause of the bolt coming loose. I think the nut had been coming loose for some time and the sudden jerk of the fall over caused the A frame to "spring" loose from the pivot assy.
This is something that others might want to check on their machines although I don't know why the locking nut would come loose. My K only has 24k on it. Still an ounce of prevention, etc. You can see the allen bolt I'm talking about in the parts cd breakdown. It's underneath the A frame and on the left side when facing the front of the bike.