does anybody have an idea what it could be it only starts to grumble when its warm could it be the bearings or diff is it ok to drive

When was the last time you checked the level of oil in the diff?

Start looking at the simple solutions first.

Ask some peeps tomorrow eve Lardy.

Capri diffs normally seep oil from the rear casing as it corrodes and rusts away. You may find that if you remove the diff cover and clean it right up you'll see small pinholes in it and consequently oil leaks away from these areas. The rumbles are normally attributed to lack of oil so whip the cover off, parrafin clean it and inspect it. If it looks ok, the oil in the diff may not be the right oil (or even diff oil at all). Put a new gasket on, the right oil in it, inspect the teeth of the cogs for wear and i would replace the diff bolts and hey presto. If it's still rumbling it may be the rear wheel bearings or if your motor has a split propshaft (i don't think it has) it could be the centre bearing of the prop.

Ill show my technical prowess now - I thought you filled it once and forgot about it i will top it up tomorrow thanks ,and i do have a two piece prop ill check it but seeing as it only does it after a few miles must be lack of oil

right topped up with diff oil just see tonight if it stops rumbling checked the old haynes manual today and what you have said is what it says in there as regards noise and lack of oil and i thought those books only had nice pictures will see later if it has rectified the problem