I had the same problem. It drove me NUTS.
It ended up being my seatbelt latch (the part you plug the seatbelt into the plastic thing). Every time I hit a bump or turned, my body movement would slightly move around the seatbelt latch, resulting in the area it was mounted (I think it was on a threaded bolt..) to click. Part was going to be replaced, but they tried spraying lube first, which did the job.
I know I sound crazy, but the click would travel up and bounce off the roof, so I kept feeling like the sound came from the roof. Wobble your seatbelt acceptor/latch/thing and see if it clicks in a similar fashion. Might not be your issue, but I thought it might be yours since you say it happens whenever you turn.