From Moss Motoring 1985
Kevin Edgley Duncan, OK
A random ‘knocking’ or ‘clattering’ sound at idle on MGBs with the 3-bladed fan may be due to bad rubber grommets in the fan mounting holes. The sound is similar to that made by a bad water pump bearing.
To check the fan, rotate it first one way and then the other (engine off). If you hear any metallic contact replace the grommets (Moss #282-830). You’ll probably have to remove the radiator to get to the lock tabs. At this point, check the pulley mounting flange of the water pump for any wiggle (as you would wiggle a tire for a loose wheel bearing). Any play and/ or knocking sound indicates a bad water pump bearing, even if it’s not leaking.
The clattering at idle from either source is virtually indistinguishable, so both tests are worthwhile to do.
Ed note: Check the fan grommets, water pump, and fan belt as part of your regular tune-up routine. It’s a good idea to have a spare water pump on hand.