Random clanking noise driving me to despair

easybiker

My trusty old Specialized Hardrock has acquired a new feature. It makes random noises. By which I mean completely random, not related to chain, pedal or wheel revolutions. The noise is a single clank, like someone tapping the metal frame with a hammer. It does it on average, probably about 10 or 20 times per mile. It does it whether the ground is bumpy or dead flat, whether I'm going up hill or down, whether I'm pedalling hard or light, and even when freewheeling. There is nothing to be felt through the pedal, no momentary chain slippage. Cleaning the drive system makes not the blindest difference. The bottom bracket is fairly smooth with only the tiniest looseness. I'm fairly sure it's not that. My gut feeling is it's at the rear rather than the front. Any ideas anyone?

The Rookie

Tried shaking it in case something is loose inside the frame?

Checked wheel bearings for slop?

easybiker

It's louder than something loose inside the frame, and it doesn't happen when manhandling the bike, turning it upside down etc. The wheels don't seem to have any play in them, if that's what you mean by slop in the bearings.

apple eater

Does it happen if you freewheel as well as when you pedal?

I had noise, not a clank as such but a very annoying noise similar at times to a clanky type noise and it turned out to be loose spokes in the rear wheel.

cookymonster

sounds like worn bearing cones ( cones pitted causing bearings not running smooth ) to me if its oldish bike , have you ever had the bearings regreased etc ?