Is this a noise that ONLY occurs when you play notes? Or does it sit there making this hum all the time it is turned on?
If it only occurs while playing, then it suggests a speaker with a rubbing voice coil. Also possible are a cabinet buzz - something is loose and rattles.
If the buzz is more a hum - sounds like when you touch your guitar cord - then it is electrical. Could be as simple as a loose ground, or could be a problem with the power supply, like a failed filter cap.