There are many causes and most were stated above! Servo type? Depending on the type, going to a 6v pack will cure it. Also increase your amp rating on the pack. 1500+ should do the trick. If your leads are long(24" or more) then try heating the leads and twisting them into a spiral. This prevents noise from traveling. You could also install a noise trap(ferrus ring). It could be location, tx battery pack, metal to metal hookups, ...etc.