You have a fan and a hard drive. If your movie or game runs from CD or DVD, that's another moving part. There are also anomalies such as "sympathetic vibrations" that can occur. These would be other parts that pick up the vibration of moving parts and vibrate on their own. The intensity of sympathetic vibrations can vary but I won't try to explain the dynamics here. When you're playing games or watching movies, disk activity and cooling need can increase. The fan speed may increase. If it's speed matches that which can cause vibration in another part such as the case or even something internal, you can get the noise from a non-moving part.
It's a compaq C700.
The bottom left hand corner is making this sound. It's not all the time but seems like whenever I do anything like play a game or watch a movie it'll start making it.
This is a recent thing so I'm a bit worried. Just started 2 days ago. It's similar to the regular noise it makes but it sounds different now. Not as loud and a bzzz sound.