I am getting a “tock, tock, tock” sound out of the back half of my bike. It occurs only occasionally, and never lasts more than about 5 seconds at a time. It beats in rhythm with the turning of the rear wheel, but it stops if I stop pedaling. For a while today, it would “tock, tock” for a few seconds, then quit for about 10-15 seconds, then return (I was pedaling the whole time).

The bike is a 1983 Trek road bike (Reynolds 531), with Suntour Superbe components. The wheels date from the late 1980s, but have only about 7,000-10,000 miles on them. They have sealed bearing hubs (Mavic), with a 6-cog threaded freewheel (Shimano-recent purchase). The chain is probably nearing replacement, but neither it, nor the derailleur, nor the cogs appear worn or bent.

I also currently have fenders on the bike, but I have carefully checked that nothing is close enough to rub. As near as I can tell, everything else is in order—no rubbing, or parts knocking together. A friend said he thought it sounded like rubbing sound, but I think it sounds more like a “tock, tock.” I would almost say it sounds like the "ping" that spokes will make as they seat themselves on a freshly-built/overhauled wheel. But, yes, the wheel is true, and the spokes have even tension.

To me, it’s a mystery. Any clues???

With my many thanks in advance,
Jamie