Classic bare brass tenor bought a couple of years ago now, and only recently had this fault pointed out to me by a sax tech a few days ago.
The bottom C key protector is soldered a little out of center. When releasing the C key after pressing, it doesn't hit the protector center on, where the felt is. Instead it's metal to metal and makes a clear and distinct clinking sound which I'm now very aware of after having it pointed out to me.
It's not a major problem obviously, and the horn still plays fantastic and I wouldn't dream of parting with it. My tech says it's a great horn other than this, and it's part of what I assume might be a quality control issue.