Having an issue with sound in my iMac.

I have a 2 year old iMac, I want to say it was the last of the white ones, but I could be completely wrong.

Here is my problem. The sound, when listening to music, seems to fade in and out. It's subtle, but annoying. At first I thought it was just a bad cd rip, but it was doing it on things I had bought from itunes. Any ideas?