Sounds like a joint or cartilage problem. You could try taking glucosamine supplements. They promote healthy joints. Also make sure you are warming up before you start lifting. I do 10 small arm circles, 10 medium, 10 large, forward and backward. Stretch your shoulders, do a couple of real light warm up sets. If you aren't warming up try that initially and see if that helps. It's really important to warm up, especially on shoulder exercises. It wouldn't be a bad idea to spend 10 minutes warming up if you need to.
If you are real concerned about it you could see a sports medicine doctor i guess