Won't crank?

Ok, next time this happens I'll whack the starter and see if that frees it. I'm pretty sure this starter is only about a year old, the one I had before this loosened up and the front of it snapped off.

There are no shims on this starter at all...

69signetv8 - When I hit the start button, it just clanks once, sounds like the starter is engaging. I'll have a look in the tranny to see if there's anything floating around in there.

moper - the battery is disconnected when it's not in use, and both the terminals and the posts on the battery are clean. Every time this happens the car is cold, has either been sitting overnight or just sitting around for a while. Once I get it to start, it will continue to fire right up.