New Windows 7 system: When I plugged in my headphones into the front audio jack, I immediately started hearing a buzzing and whistling noise in the earpiece. When I switched to the rear audio port, this noise went away. MP3s played clearly on both ports albeit with the buzzing noise in the background when using the front port.
A kind gent on IRC informed me that this sometimes happened if I was using a wireless mouse or keyboard and it was true, I was using a wireless keyboard. However, unplugging it did nothing to stop the buzzing. But I did have a wired mouse also plugged into the front USB port. I removed that and voila!, the buzzing vanished.
When I reconnected the wireless keyboard adapter back in, I could hear some faint static unlike the loud buzzing with the wired mouse. I've now moved all permanent USB fixtures to the rear USB ports. All is now well in Denmark.
Hope this helps!