Faint buzz in speakers

I have had this problem twice. Once, it turned out to be proximity to a router. When i removed it from the room, the problem went away. 

The bigger issue turned out to be a defective toroidal transformer in my amp. The manufacturer replaced the transformer, and the problem ended. As someone said, sometimes our power, or the system doesn't agree with one or more of the pieces. But sometimes, it really is a definable local problem.

Someone once said to me many decades ago: "The goal is not the absence of noise, but the presence of music." Since then, I don't sweat things I can't hear from my listening chair. 

David

I noticed that sometimes it's a light rheostat (sorry, dimmer switch) making noise in the circuit. However, it's usually a cheap charger that is the culprit in the circuit. 

I noticed that sometimes it's a light rheostat (sorry, dimmer switch) 

Any self respecting audiophile should get rid of all dimmer switches in the house.

Or, that self respecting audiophile could simply make sure all lights with dimmers are turned off while listening.

If this was my system, it would be rewired, use different outlets. If anything is 2 prong, instead of 3 prong, swap the cord around. On 2 prong power plug, swapping the plug around can have profound sound differences. 

Of course make sure there are no lights turned on, on that circuit, signal cables are away from power cables.

Might also be your amp needing some service, like bias adjusted, or a component going out of spec.