I have a 2008 2.0 TDCI focus

On occasion (every couple of months or so) my gearbox/clutch? starts

making a grinding noise for about a day. This has now become like the

sound of a big bag of spanners!

It only occurs in neutral or with the clutch in (whether in gear or not) If

under load it totally goes away. The car still engages gear and the clutch

still works perfectly so no problem driving until the other day when it

started while changing through the gears - the rumble continued until

there was a load bang as if something had been thrown in the gearbox

on making contact with something moving. The car continued on perfectly

with no more noise. Came back a week later sounding like spanners

again! Gone again for now and to listen you would never know there was

a problem. Anyone know if this sounds like the clutch or gearbox or

something else (it's been doing this now for about 20000 miles but never

with such a bad sound as recently). When it occurs you can feel a bad

vibration through the clutch pedal. Not sure what it is so I've never got

the timing right to get to a garage when it occurs for them to listen to it.

Any idea??? Help appreciated.