E207WFP, random cracking noise

My LCD monitor (Dell E207WFP) is about 3 years old, and has started randomly making cracking noises, it can be up to half a dozen times an hour (or less). The noise is similar to pressing on the plastic surround of the monitor. At first I thought it might just be the plastic expanding when things get hot/cold, but the noise will occur whether it's in-use, or up to 10 hours after it has been switched off. The monitor seems to work without any problems otherwise, and there is no distortion to the picture when the noise occurs. As the computer is in my bedroom it's a real pain when trying to get to sleep. I've tried various things which have made no difference:

- Removing all other hardware from around the PC and monitor
- Plugging the PC and monitor into a different plug socket
- Unplugging and checking all the plugs between the PC and monitor
- Reset the monitor settings

Just to make things slightly more complicated, I'm sure I've heard similar sounds (though not as frequent and probably unrelated) coming from the main PC unit.

Any ideas?

Thanks