Digital servos may hum even at neutral, usually with a little fine tuning of the trims and the linkages you can get rid of it. Make sure you set the end points on the radio correctly so the servo does not hum loudly at full lock, it could damage the servo.
The speedo lurching sounds like interference from the servo coming through the receiver. It may be related to the speedo, or it may be related to the servo. The Ansmann radio is made by Sanwa (I think) so should be decent quality.
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