Thanks! My Kala uke has these cheap enclosed tuners on it. After changing to new strings, it developed a very loud buzz on the C. I changed that string, no good. I shimmed it at the nut and saddle, no good. Then I checked the nuts that hold the washers down on the face side of the headstock. Sure enough, it was loose as a goose on the C tuner! That's something I'll watch out for from now on!
it doesn't only happen to cheap tuners. I all the sealed tuners on my ukes have nuts and some have vibrated loose at one time or another. It's the first thing I check with a new buzz.