I have a number of 66s and 2 99s. Both of the 99s make a bit more sound than the 66s but not anything objectionable. The do need to be kept clean and oiled, that is the truth.
Also the bobbin case hold down has an adjustment between the springs of the hold down and the case. If someone has taken the screw out thinking that screw was to remove the bobbin hold down this adjustment will be messed up and the machine will make a clacking noise as it sews. I'm working on an early 99 in a knee bar case right now that had been bubba'd. I've got to dig out the specs and readjust the bobbin holder to quiet it down.
Joe