The servo in my wheely king has been making a noise for a while now so I decided to open it up and have a look. I think the bearing is bad, if you look closely you can see a few of the balls in the servo top. There is also a little rust and dirt in there. This is a Hitec 645 and it was protected with plasti dip and a rubber o ring on the main gear. Sorry for the blury pic, I'm no photographer.
This is what the bearing looked like once I removed it
This is what the bearing looked like once I removed it