Audio Systems: Pop, possibilities, amp

Question
What would make my speakers pop and hum?

Answer
There are many possibilities.

Send me the exact make, model and age of the amp/receiver and the speakers and if the hum stays constant or goes up and down with volume.  And, is it only in one input, such as radio or cd, etc.  Let me know as much as you can and I can give you a better answer.

Cleggsan