What kind of noise
? Is it just a smooth "whining" noise
that varies with boat speed (normal when free wheeling) or is it more of a grinding/mechanical noise (not good)?
The big down side to sail drives of course is that they are mechanically quite complex. Fortunately, in my experience, the Volvo
drives are quite reliable, but when they do need work, other than just normal simple maintenance
(like changing the prop shaft seals) I recommend having a trained Volvo tech do the work -- specialized knowledge, tools (a long list in the service
manual), and parts
are needed to do it right.
These units are also "one off's" so to speak because each one is adjusted differently with varying thicknesses of shims. So, a slightly louder whining from one than the other would not concern me so much (and there's not much you can do about this).
If you are lucky, it could just be damage or fouling around the prop shaft (relatively easy to repair/replace). This can be checked easily next time you haul or even just dive it an look for bits of stuff between the prop and housing. Check for lateral movement of the shaft on one versus the other by pushing against the prop.
If you suspect something more then I suggest consulting with a qualified tech (digging into the guts of a sail drive is not a job for the average joe mechanic).
Also, what year model are the drives/engine? If you are even luckier they might still be under warranty (Vovlo once replaced an entire sail drive unit for me under warranty -- including labor -- that's a really ugly tab if not under warranty).
Any impact or fouling (like fishing
line) to the problematic drive lately?