Okay, it was just a thought, and another one is whether the gear shift cable was disconnected during the operation, and whether it had been correctly re-set once the saildrive was back in place. If the job was done by boatyard staff, it could be worthwhile to complain and say that the system makes an unusual noise while in reverse. On my boat when in neutral the gear shift lever on the saildrive points transversely port and starboard across the boat where it is at 90 degrees to the fore and aft line. I’m just wondering whether in reverse if the set up is not correct, whether a “not fully engaged reverse gear” would make the noise you have mentioned, or whether a reverse gear selector under excess pressure from the gear lever might make that noise.
It’s also worth checking around the engine if you had boatyard staff do the seal change, to ensure that all is exactly back in place as it should be. On mine the yard staff managed to reconnect the alternator incorrectly, and fortunately for me it was noticed pretty much straight away as the tachometer had stopped working. Fortunately too I’d taken several photos of areas that might get disturbed during the job and had a photographic record of where the alternator wires should have been connected.
