My arm was quite bad at one point. Every time I rotated my forearm back and forth my elbow used to click. This would come from within the joint and as you can imagine was quite a concern. Went to the doctors who referred me to a physio. She told me pretty much the same as everyone else on this post.
I purchased a gell filled mouse mat and keyboard support, which fixed the click within a few weeks. The thing was, I was still having pain in my forearm. So after some discussion at work I was advised to get a grip master (or whatever them forearm training things are called). I do this for about 15 minutes every other day (1 minute of use, 1 minute rest) and all the pain and annoyance has disappeared.
From the post here, I’d say this is quite a common problem amounts people who work with a keyboard and mouse all day.
Also, work offered me a risk assessment which entails someone coming over and checking the setup of your desk and chair. Would most probably be worth getting one of these as well