Most likely it's from expansion and contraction from heat from the lights.
What else could it be?

That could be it. A few of the bulbs are not exactly centered in the recesses. They may be too close to the edges, causing the noise. I think we will have them looked at again, and hopefully fix whatever this is. Thanks so much

Sounds like something in these recessed lights is not securely fastened down, so that when they heat up, something pops loose. It odes happen after they have been on for a while, and have heated up, right?

You should check these out, and see that all the fixture screws & connectors are completely fastened. Probably not a life-threatening problem, but if left unfixed it is likely to shorten the life of these fixtures or bulbs.

Your right..they had been on for a bit. I will have all of that checked, just to be on the safe side. Thanks so much for your time.

I think all you need to do is to fasten all the connectors securely and make sure the recessed lighting fits snugly in the hole. Hopefully you have already fixed this problem.

David - http://www.savoydoors.co.uk

We did. The electrician came out, and adjusted all the lights. Have not heard the noise again. Thanks so much for you time.

How old is your house? It could be a factor.