How often do you get the noise? Sometimes an amp will pick up your cell phone "checking in" with the local tower. The best solution is to keep your phone away from your rig.
You might also pick up noise from things that emit RFI, like fluorescent lights, dimmer switches, AC motors, HVAC systems, refrigerators, computers, CRTs, neighbors who operate amateur radio, and anything that kicks out RF. Hunting down the culprit can take some time, but look around and consider moving your rig.
You might be getting some noise from your source. If you are feeding the amp from a computer, the noise could be coming from inside your computer. They're noisy. If you have a DVD player, try connecting it to your amp and play a CD. If the noise goes away, it's your computer's fault.
Finally, you might have a ground loop. You can try plugging it into different outlets to see if that helps. A "cheater" plug (one without a ground pin) can also isolate a ground loop. I don't recommend cheater plugs for anything but testing since the ground is a rather important safety feature. If you do find a ground loop, I'd recommend finding an inexpensive isolation transformer.
Please let us know if you make any progress with the noise and what worked - it would help out others searching for information.
