You Already Knew it was A Clicking Problem

I live in a Big10 college town and used to work for a taxi service here in town as their mechanic. The majority of our fleet were 2nd gen, a couple 8's, several 12's and a 14 and a 15 or two. Anyway, now I own an 09 and I'm hearing a click, the same click that plagued me as their lone mechanic for 3 years. It sounds very similar to a bad CV joint, but a little higher in resonance and shorter in length, and sounded like it was coming from the driver side inner joint. I replaced several CV axles (it was a multi car issue, just about all of them) and the click would disappear but come back within a week. Before I ever managed to figure out whatever was actually going on, Uber came to town and you know the rest, the doors were shut within a year, though I saw the writing in the wall before they did and got out. So when I left the guy in the shop next door to us took over maintenance of the fleet, and I ran into him not long after. He had figured out the problem, something to do with a ring gear, but it was a few years ago and I don't really remember. Has anyone else had experience with this issue? Can you tell me anything about it?