New bottom bracket on my brother's bike.
We went out for a 20 miler today and a little after 10 miles I noticed his bike making an odd clunk. After stopping to check and finding no obvious cause (no play, nothing loose) we continued but it got steadily worse until it was a thump that reverberated through the frame twice every revolution of the pedals.
Grabbing the cranks and manipulating them I still found no play and no feel of grinding but it did clunk once - which I didn't manage to get it to repeat.
I've done a swap as I happened to have a new one the right size in my collection of spares and all is now silent again.
Now it's out there is a very, very slight grittiness when turning the spindle. I'm surprised such a small fault made such a lot of noise.