It sounds like it's probably a grounding issue with your case/motherboard/jack. I have an Antec 1200, and when I connected to the front panel, I would get a high pitched squeal coming through. They sent me a new front panel and it went away. Some fixed on the net were to sand down that front panel's ground due to it being painted.
It could be capacitor whine too, usually is related to dirty power somewhere.
Some people are able to get a headphone extension cord, with a volume control in it, radio shack sells them, and sometimes that is enough to kill the squeal.