Many servos tend to hum even with the slightest amount of binding present. Of course you should try to make the pushrods move as smoothly as possible, but sometimes it's really tricky to remove the humming altogether. Make sure there isn't excessive binding, because that might drain your rx battery quicker than you want. Don't lose any sleep over the occasional humming, though. If you think about it, most of your servos will be under load all the time in flight anyway. For instance, if you have to trim your ailerons a few clicks during your maiden flight, then your aileron servo(s) will be under constant load in flight from then on (think about it, and you'll see what I mean). They'll be humming up there, but you won't hear it. I'll probably be flamed for this, but...
