Speakers buzzing

I have buzzing problem with my speakers. I can get them to stop buzzing but I am not sure why it works. I think it is some type of interference but I am not exactly sure what. 

I have a Yamaha receiver in my detached garage with speakers wire running underground to my speakers in the yard.

I know my speakers are fine, I have tested elsewhere. My receiver is fine, I also tested elsewhere. One of the final things I tried was, leaving my receiver in the garage and run an extension cord to a different location speakers still buzz. I unplugged my garage door opener, all my chargers, extra refrigerator, florescent lights they still buzz. When I cut all power to my garage and plug into extension cord plugged in somewhere else they stopped buzzing. Turned the power in the garage the buzzing started again.

Is there anything I can do be able to keep the receiver in the garage or is it likely hopeless?