Could it be that the noise does not really disappear but gets lost between other noises when you increase the rpm?
To quickly find out if it originates from an u-joint, turn the outdrive from left to right at idle rpm. If the noise doesn't change substantially, the problem is in the lower gear case.
Unscrew the bottom plug a few turns and see what comes out: water, milk, oil with metal scrapings etc. If it is just clean gear oil and the level seems OK, accept the noise for this boating season.
cogito, ergo sum (Descartes' credo)