So I also made an attempt at making those servos a little bit more quiet. I opened one of them, and filled it with some servo grease that I ordered from China some time ago. Assembled it back together, and...

Turns out the servo is perfectly quiet now. Unfortunately, that's because it doesn't move.

Turns out that the smallest gear -- the one near the motor -- has to move with such ease, that even a little bit of grease makes it completely stuck. It took me about an hour to clean all the grease from those gears, and the servo works again. As loud as before, though.