I've never used a 4721 so I don't know if they exhibit the same problem but I do know that they are the exact same servo except the 4721 has nylon gear whereas the 2721 has metal gear.

The 2721s are sorta wierd in that they always seem to work fine when you first put them in an airplane and for a while thereafter. Then, one by one, they start this chattering/vibrating phenomona and once replaced with 8411s all of the problems go away.