Microphone Problem

[JonDHayes]

To start off, i have a 20" 2007 iMac. There are not many problems with it other than that it runs pretty slow. But what I'm having trouble with is that I'd like to voice chat with some friends while i play games using an application called Mumble. My problem is that on my old headset, there would always be this constant unbearable background noise coming from my mic. It doesn't only happen on this application, it happens on skype and teamspeak too. And it wasn't the headset because it works fine on my iPod. But still, i bought a new Astro a30 headset, thinking that it might be a compatability issue, but still, the same issue keeps happening. I would like to know if there is any way to resolve it, or if it is caused by the computer's audio/mic input. If that is the case, does anybody know if i can buy a usb to female 3.5mm jack, or im im stuck to using my ipod. Thanks.