It is quite common for servos to move slightly when you turn off the transmitter. It is always suggested you turn the receiver off before the transmitter. Some people turn the receiver on before the transmitter to check for interference. I don't like doing it because some receivers will thrash the servos violently if there is a tiny bit of interference, but as soon as the transmitter is turned on, range checks work beautifully, and no problems are experienced in flight.

The easiest way to find out what is binding, unplug all the servos but one. Move the stick associated with that servo, if there's no buzzing, after you stop moving, there's no problem. Continue this way till you know which servo is experiencing binding. If the servo has two items on it (ailerons or rudder & nose wheel), disconnect one pushrod by the servo while testing the other. It will require some time, but is well worth it in the long run.

Keep in mind that if you have large (heavy) control surfaces, sometimes their own weight will cause sufficiant load to make a servo buzz a slight bit.