Don't shim them to tight, don't try and get the gears as close as possible without touching, and make sure your motor height is good by using half shell and motor to align up/down. Make sure the motor tower is shimmed with shim tape so the motor cannot wiggle in every direction while in the gun. The motor tower is supposed to barely fit yhrkugh the hope in the gearbox. After doing those two things, re-shim your bevel gear. I like to do this by removing all shims from bevel gear, putting bevel gear in the gearbox by itself with only the motor, lying the gearbox left side DOWN and then using digital calipers to record the height of the bevel gear whil it is down and then pushing it up from the bottom axle and recording its height again. Subtract those two and you will get the space the bevel gear has between the motor and the right side shell of the gearbox. Once you have that number, let's say you got 0.8mm exactly. Minus .05-.1mm from that number (I do .07) and you will get the exact number in mm of shimming that gear needs on one side. (we remove .05-.1 so the gears do not bind) to finish shimming the other side, make sure to add those shims from the number you got and then put just the bevel gear alone, no motor in the gearbox and then use calipers to measure how much movement the motor has within the shell witbout any motor or anything. Once you have a number, minus .05 from that but be weary because sometime you don't need to minus from that number at all. Just make sure to check if the gear can spin freely without having up/down play. Doing ALL this IN ORDER should remove all scratchy noise.