getting a funny noise while recording with the "mic in" jack

I posted this in the hardware forum, but thought I'd crosspost here, just in case there's a software fix...

When using the "mic in" jack I'm getting a noise. It only happens when I have a cord plugged into the mic in jack, and it happens to any cord I plug in, regardless of if I have a microphone plugged into the cord or not. The noise is a somewhat rythmic hum, repeating everyone 1.25 seconds, approximately.

I'm an audio engineer for radio broadcast, and I intend to use my Aspire One to record and playback audio, using the program Audacity. I've tried plugging in different microphones, cords, and audio sources, and I consistently get the noise. I've then tried some of the same cords and audio sources on another computer and NOT gotten the noise. I've also recorded some audio using the built-in microphone on the aspire one and NOT gotten the noise, so it seems to be only on the mic in jack on my new computer. The noise is directly affected by the volume setting on the recording section of the sound software on the computer... the louder the recording level is set, the louder the noise is, of course.

I have an MP3 recording of the noise, if anyone wants to hear it and help me diagnose...

Forgive how long this post is, but I wanted whoever reads this to know that while I'm not a computer expert, I AM very good with audio, and troubleshooting audio problems, and after checking and doublechecking and triplechecking, whatever problem is happening is centered on the "mic in" jack on the computer, not with any other audio source, cable, or my recording software.

So, has this happened to anyone else?

Thanks!
Ted