On 25/03/2021 21:58, T i m wrote:
I started to hear a regular 'clonk-donk' as cars were driving over
something in the road, like a bit of chipboard shelf but it then
persisted for around a couple of weeks.

Yesterday I investigated when we took the dog for a walk last thing
and it appears at least one manhole cover has been lifted ... or has
worked loose(?) and is now rocking about 5mm when you stand on it?

This is just one of the single piece, all metal (cast iron), slightly
rectangular ones with just a BS number on it and no clues to what
utility it might serve but from it's location I'm thinking it might be
the sewer?
....

That can be caused by the brickwork of the hole itself starting to fail,
causing the support frame to move. Repairing that requires major works
of the sort that normally has to be planned months in advance.

It can also be due to part of the cover or frame breaking off and
allowing the cover to rock. That can sometimes be alleviated by pouring
sand in, but only if it stays in place and doesn't trickle down the hole
because it is part of the frame that has broken.

--
Colin Bignell