Volvo S60 transmission noise

Car has 200k miles and clutch and flywheel replaced at around 140k. Noise from gearbox area when decelerating at 40 and in most gears except 5th Fine (no noise) cruising at high speed. Sounds like an old-fashioned timing chain or loose tappets - but doesn't happen when clutch disengaged (so not to do with engine). Have driven 7k miles on itsince noise started- garage didn't really know and suggested fix gearbox (replace/re-condition) - but not with a lot of conviction.

I like the car having owned since new and reluctant to abandon it yet. looking for explanation and not too costly solution (or is this silly?)