Yeah, the demo version inserts a distorted hiss sound every few seconds.
Sounds like you need the full version

If you need to hear Amplitube in action, just go to my lesson page.
About 99.9% of the guitars are straight line in, then processed later.

If you want to evaluate software without being limited by demo's, you should should usenet in newsgroups such as alt. The program is fully functional with no time limit.
Once youget the hang of it, erase it from the hard drive and buy a new one.