Popping Sound from light fixtures

Hi,

For the last few months, we have noticed that two of the four recessed light fixtures in our kitchen make a periodic popping sound. It can happen when the lights are on, but most often it happenss for a couple hours after the lights are turned off. These fixtures also seem to burn through bulbs in just a couple of months. When we take the bulbs out, the glass has come right out of the metal base. The two parts are only connected by the bulb's internal wires. We, being a little lazy, have just gotten used to it, but it needs to be addressed now that our house is for sale. We live in a 4 year old, one story single family home. Any ideas?