buzzing?

Hi, Kit. Your system could be buzzing from many different things. I will give you a few things to try, hopefully it helps some.
First I will assume that all of the components in your system are equipped with a grounded plug. One thing that might be causing the hum is if you have different items plugged into different wall plugs that may have a different ground in the buildings electrical system. What happens here is that current can travel through the earth from one ground to the next creating a loop of current and a subsequent hum. An easy way to eliminate this is to: A) plug all items on to the same electrical ciurcuit, or: B) to drop the ground connection on one end of your patch cables that carries the signal from one unit to the next.
A power conditioner may also help since another electrical device plugged into that circuit such as an electric motor from a fridge could be causing your hum. Florescent lights can sometimes be a culprit as well.
Barring all these things there is always the possibility that one of your machines could have an internal electrical problem that may need repair.
Hope this helps.

Buzzes, squeaks and squawks

That's good advice from Vinniesrs. Another item that may cause buzzing is a light dimmer switch. Home theater systems are often affected by this. Try switching off any computer monitors that may be nearby. I find this to be a source of noise in my own project studio. Sometimes if you change the monitor refresh rate on the desktop settings, that can help.

By process of elimination, try unplugging a few things and listening to them on their own (PA only, mixer only etc.).