OOW
I'd like to suggest that when you lift the xmon that you also pull the freewheel and check it. The symptoms you descirbe fit perfectly to a freewheel failure soon.
How long since the xmon has been overhauled or a mid point (600hrly) done. One thing for sure is that if you cannot vouch for the exact hours then you should check it. The non negotiable exact time called up inspections are for the grips and freewheel on any '47.
see if it is a lighweight or medium weight freewheel. if it is either try to get a heavyweight unit installed.
you may be able to check the worksheet of the last xmon overhaul, to see if new rollers were installed or at least the actual miked measurements of the rollers and ring.
when the freewheel fails the engine goes straight to valve bounce speed, if you are lucky the fan and or fan belts don't take out any of the flight controls and you descend immediately. it is a (VERY) frightening and disorientating sound.
But as Nigel says it may just be the clutch which could be because of a loose retaining nut, which means check it straight away afore you wreck your fan drive.
tet