Yes, digital servos usually hum at center in a constant effort to maintain position. Just the friction of the tires on the race surface at start, or the little friction in your steering linkage causes enough resistance for the servo to sense the need to correct. Once running on the track it goes away. You can minimize it by freeing up your linkages, but it isn't a major issue unless you leave it sitting like that for a LONG time. That might cause heat build-up in the motor and some damage to the servo. Normal everyday use won't be a problem at all. I have yet to find a digital servo brand that doesn't buzz to some extent.