Why doesn't STP allow me to save sometimes. I need some helpful hints on how-to.

When I play a musical clip from the library, sometimes (especially when I leave it run for a while as I'm trying to listen and hear how it plays with different pieces of music) I'm getting a hum that grows and grows - a window opens up, I click on it and after a few seconds the hum dies - it sounds like the speaker will explode anytime :-) What is this phenomenon?

What are the simple basic steps of using STP. I know how to create wonderful soundtracks by simply using the tracks window. I need to go beyond this. Can you tell me some cool features of STP? Thanks.