I heard the slapping sound last on Saturday, rode it Sunday and today and heard nothing. I'm fairly confident it was the primary chain just at its loosest point before being ratcheted up to the next step on the adjuster. Knowing how it works makes this seem like the most likely scenario, although I'm surprised it would allow itself to get that loose between adjustments. When I checked it Sunday AM it wasn't loose enough to slap, IMO, so maybe it had adjusted itself before that time. Once on a step on the adjuster it can't come back down.

When I tried to isolate the sound I put my ear directly to the outer cover, without actually touching it when hot of course, and the noise was clearly in the area where the chain would slap against the housing. There isn't anything else in that area. I'll keep my ears open, though.

 

No, I didn't get that detailed with it, as I only wanted to see if it was severely loose, which it wasn't. The mention of 5/8" was a guess, as the chain isn't close enough to measure when accessed from the derby cover.

 

I had an M6 on my '96 RK and it lasted 60k miles before a spring broke. Lacking confidence in it after that I reinstalled the stock tensioner, then later added the HD unit when it first came out for retrofit to earlier bikes. No problems with it up until I sold the bike about a year later.

Bikes with the auto-tensioner have no inspection cover, so I can only peer through my derby cover to see the chain. You can see well enough with a flashlight and long screwdriver to know if the chain is too loose. The sound did not originate from the front of the primary cover, but rather toward the middle where you'd expect a slapping noise from a loose chain. Anyway, like I mentioned, it seems to be gone for now, so I presume it adjusted itself. This noise was sporadic, maybe once every 5-10 sec. when at idle and stopped when the engine was revved beyond that. This was identical to my memories of older bikes with loose-chain slap.