Never forget to re-seal the gearbox with RTV right after you’ve got found the correct mixture. Even although they are called “gaskets”, they won’t seal your gearbox on their own.
If every little thing seems to be normal, try changing the gear-mesh. This is completed with the mylar “gaskets” that sit among the primary gearbox circumstance and the enter/output caps. Attempt introducing or removing some of these spacers till your gears have roughly one/8″-one/four” of free perform. Specialists can order replacements ($1-2) in various thicknesses, but we advocate the fast and filthy method of chopping your personal out of a sheet of mylar.
“Groaning”, “creaking”, or “popping” all seem type-of related above-the-mobile phone. The very best point to do is 
to disassemble your gearbox and just take a look at the gears and bearings. The situation w
ill typically current by itself in the way of broken/lacking gear teeth, toasted gears (like that photo on our property web page), or blown bearings.
My Gearbox is Groaning/Creaking: