I'll add my 2 cents about gear mesh as well...
When new gears are installed in new devices, there is a precise locational relationship between mating gears. Any deviation from those locations results in gear wear and noise. Ideally, the form and curvature of each gear tooth should cause it to roll across the face of its mate, with no sliding, rubbing, friction or mechanical interference. Notice, I said "new" gears in "new" devices. At best, in our hobby, we may have new gears, in old devices. Achieving a reasonable quiet gear train then becomes a trial & error affair. The form & curvature of the gear teeth have been deviated from by wear. The location of each gear, (gear spacing), has also been modified by wear. What to do?
1. Realize that gears that have been running together for 100 years have worn to the extent that their shapes now compliment each other. (Like a well-worn shoe that has conformed to the shape of your foot after years of wear.) While the shape of the gear teeth and their spatial relationship may be very "incorrect", they may still operate rather well. As long as each gear still has some substantial amount of material remaining in its teeth, (meaning it's not about to strip), this can be an okay situation. In these cases, if the gears are to be removed for cleaning, great care must be taken to re-align them exactly as they were. It sometimes requires a fair amount of trial-and-error to find the "sweet spot".
2. Replace a worn out or stripped gear, either with a used one, or a new one? Keeping in mind the above, any "foreign" gear, new or used, that is mated to another used/worn gear, in a worn device, can ultimately produce noise. With the analogy of the well-worn shoe, recognize that while the shoe has conformed to your foot, so too did your foot conform to the shoe! Think of the discomfort of a new pair of shoes. All we can do with our replacement gear, is to adjust their alignment as best we can, (assuming the motor even allows for that). Time and use may quiet things somewhat... or possibly not. Take some confidence however, that when we use gears in decent condition, we at least know that they are strong and not about to strip and do huge damage. The noise may be irritating, but even though it signifies an improper mesh, it will take most of whatever lifetime we have left before it even begins to be an issue. These are not like cars that we drive 30K miles per year.
3. Replace both gears of a mating pair with new gears? This is a better option. Along with repairing the motor housings and/or bearings to re-establish proper gear spacing. In other words, rebuild the motor. Really, while this is sometimes necessary, it is also extreme and costly if you can't do the work yourself. With most pairs of mating gears, one gear, (the one with fewer teeth), gets the lion's share of wear. Replacing only that gear is usually sufficient. Note also, that for the most part, phonograph motor gears are very slow turning. The exception being, some governor gears. Slow turning gears have a very low potential of producing noise, no matter how bad they are. Those gears need only to be strong enough to not pose a risk of damage due to complete failure, i.e., stripping.
Anyway, I know I have restated some of Django's points. It simply means I agree with him

Also, these are my thoughts and approaches to phonograph motors & gears. Yours may vary and mine are not intended to be the only way to approach the challenges of phono motor repairs and restoration.