I am currently playing with the HS-485HB servo, and I noticed that very frequently the servo will not stop in a "rest" position after it stops moving, even when external forces applied to it are tiny. Instead it stops in a roughly correct position and continues to "buzz", while drawing more current than it should (~30mA). If I gently tap the servo horn with my finger, it settles into the rest position, the noise stops and the current goes down to about 7mA, which is what I am looking for.
Here's a video that demonstrates the problem: https://www.youtube.com/watch?v=GSLFnIpSqXw
What are the possible reasons for this behaviour?
EDIT: I acted on a hunch and added a 570uF capacitor in parallel with GND and +5V of the servo. This appears to have solved the problem! And it had to be that large too. For example, using just a 470uF helped the servo park properly only half the time.
I still don't know why that worked, so an explanation would be appreciated.