scout51

HOpe you can help me on this, Im totally new on this R/C Hobbie, and I was given a R/c Nirvana Megatech Salilboat. I used once like a year ago and everything was working fine.
Today I placed new batteries and turned on and the servo start vibrating. Actually is moving the (Arm?) slightly back and forward very quiclky (like vibrating) all this happened just when I turn it on. When I move the controls on the remote the servo did move to one side or another like is supposed to , but when not moving the controls , it always has this "vibrating" moves..
Any suggestion on what may be happening and how tro correct or fix this problem?? Thanks in advavce for your support.