Gears can typically make some noise, with worn gears making more – typically a whine. Is the lower unit full of good oil? If the gear case is empty, that can make for noisier operation, too.

Jim,
Those gears are different. It has only one ring gear & two pinion gears running the same as the D-shaft. The shift dog moves up & down for shifting & the twp pinion gears have ramped engagements. The teeth are straight cut for more noise. I would pull it down, wash everything super clean & get new bearings for it. They are common bearings too. Check for thrust washer wear too. Be sure your engagement is 100% per manual. I can scan a few gearcase pages for you too. Just send me an email jeff.50@cox.net. & I’ll get them off to you. You take a different prop too so be careful with it as it is rare & spins the other direction.
Jeff

Jim,
Here you go! propshaft not present here. See how the two pinion gears dog shift up & down, not fore & aft. Very important to get bearings & shims correct. They all spin in any shift position, just moving a different set to shift. It also runs like a direct drive motor & spins in the opposite way cause how it shifts. Those gears that Wedgie shows can be used. Take some time filing down any humps up & wire wheel the gear to remove any rust or got-yas. Be sure to get the needle bearinged gear with new needles, about $7.00 per needle bearing. There is a replacement, one side sealed outer propshaft bearing. It just doesn’t come that way. One side cover will need removed. They pop right off. Fill with 105 grease assembly grease. I get a huge syringe from the Feed & Tack store for horses. I have a one gallon can I fill the syringe using my finger, then add the plunger & add from the bottom until it comes out the vent screw clean, then cap both screws with a nice gaskets. Before assembly check the threaded holes in the case for rising threads. That leak is under looked many times. As it is tightened it feels tight but on the raised area & leaves a gap everywhere. Keep the cover gasket the same as stock as that clearance changes gear mesh contact. The smaller two pinion gears tend to run at an angle so check bushing on forward & bearing on reverse gear. The side bushings wear out too & ride up & down on the D-shaft shaft. Take your time & get everything very clean to check for wear/rust patterns. Once it is good again it will perform well for a very long time. Add a little 140 WT hypoid gear oil if it seems a little thick for all the needle bearings too. Good luck & careful assembling, it will work quiet & smoothly. Scott is the only builder who set up the gear-set like this & will function very well with good clearances. Remember though straight cut gears are noisy no matter what you try to do, but that whine sounds like a well tuned Mercury as those gears are straight cut too. Some Mercs were 1 to 1 ratio & ended up running the teeth meshing with the same tooth over & over & wore them out faster till they changed to 1.1 to 1 ratio. Let me know if I can help! jeff.50@cox.net.. I’m a Firestone nut built by ScottAtwater too.
Jeff