Rev related again. The belt tensioner is working hard by the looks of it, so I know one of the twirly bits is sticking. Have had a long screwdriver on all - the alternator is whining (so I suspect that first), but there is a tick coming from somewhere and I can't place it!
I've listened to the sump - quiet
rockers - quiet
plenum - quiet
g/box - quiet
the sound can be heard from under the vehicle or right in front, but not loudly from either side.....
I'll book it into somewhere I think ..... <gloom>