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?

Now when I've seen the workers lift these in the past (and the heavier
concrete topped ones) they seem to just hook it out one end and drag
it out the way, then drag it back in place again when done with no
apparent regard to any crud that might be stuck on the cover or the
sides of the hole. I saw them (on CCTV) working on a mains access
manhole in the pavement next to us and when they put it back in place
they obviously had issue with it seating properly / fully and were
jumping up and down on it! ;-(

Now, I've reported this one to the Highways Agency but it looks like
it comes under their 'low priority' action so I wondered if there was
anything one could do that couldn't be considered as interfering that
might help, like brushing some sharp sand into the cracks or
something? Ideally I'd like to lift it and give both parts a good
brush with a stiff brush and put it back but as it's in the middle of
the road ...

I've seen an instance where some Water board guys 'temporarily' fixed
one of their noisy manhole covers by trapping rubber gloves in each
corner. ;-)

I would have thought that if it was a stone trapped in there it would
have been crushed by now but not of it was say a piece of metal (large
nail / screw) or steel nut etc.

Whilst I really only notice it during the morning rush hour / school
run, it must be pretty loud / annoying for the houses either side?

My minor concern is that it get's worse requiring a replacement that
takes even longer to happen. 'Where there's noise there's wear', as my
Dad used to say.

Cheers, T i m