I've spent a while searching and I can't come up with a conclusion.
I have ubuntu 10.04 on a Dell Latitude D620, and up until today, I could record whatever I wanted through my microphone and it'd be fine. Now, it has started to have a terrible screeching noise..

I am assuming that this is coming from my speakers, into the microphone, back through my speakers, and through that loop. Because, when I lower the volume of my speakers to be silent, the screeching stops.

How can I fix this/why would this be caused?