Hum in speakers

Hum in speakers how do I find the problem 
A hum could be caused by many different issues. For starters, is the hum the same if you change the source?  I had a hum when I selected my turntable but not when I selected my DAC.  It turned out that my phonostage was too close to my power amp, and moving it killed the hum.

Try selecting one source component without playing music, and the volume high enough to clearly hear the hum, then switch the source and see if the hum changes in intensity or ceases entirely.  This could lead you to determine if the hum is coming from a particular component.

if that doesn't do anything, read up on ground loop hum and how to eliminate it.  PS Audio has a nice article on it.  Here is the link:

A photo of your system may be useful, and/or a detailed description. Good luck!
Does the hum come from both speakers?

Are you using an integrated amp or an amp/pre-amp combo? If it is an amp/pre combo, try unplugging the pair of input cables from your pre-amp to the amp, (AFTER turning the amp off first)  and see if the hum stops. If it does, then the hum is coming through or from your pre-amp. If so, plug it back in, turn the amp on, and then un-plug one source at a time from your pre-amp, until you hear it stop. If it does not stop when everything is un-plugged from the pre-amps inputs, the hum is coming from the pre-amp. Let us know the results.
With the Amps just plugged into the speakers the hum is still there
I would post a pic if it would help but I don't know how
Go to Home Depot and get a "cheater plug". Try to get one that has 2 spades, and no ground (long, round) connection. If they only have the grounded type, just cut off that round pin. Plug your cheater into the wall with the amp attached. If that doesn’t do it, try unplugging the cheater with your amp connected and reinserting it into the wall upside down. Post your results.  May the force be with you.
With the Amps just plugged into the speakers the hum is still there
Is the preamp disconnected or just turned off? If off disconnect the preamp at the amp end. Also is the hum coming out of both speakers? Does the hum occur all the time? Which amps do you have? Some amps are known to have a low level hum.

Do you have a dimmer switch on the same circuit as your system?
Hum comes out of both speakers it is all the time they are Sierra audio Denali mono blocks 
There is a dimmer switch which controls a ceiling fan
Dimmers are notorious in terms of dirtying up AC. Do you have a hum with the dimmer switch completely off (or, conversely, all the way up)? If not, then it's the dimmer switch. If you still have a hum, troubleshoot as the posts above advise and try to isolate which component may be the source.

You can have an electric company in your area run a dedicated line from your wall outlet to your circuit breaker for fairly cheap. That way you amps will draw power meant for them and for them only.
How old are the amps and is the hum a new problem?