Transfer switch buzzing

I used to have a buzz occasionally that I thought was the ATS, turned out it was the converter nearby. There was a set of old batteries I was hanging onto trying to squeeze as much use out of them that I could and when they were inadvertently depleted like during a long road trip with some dry camping, when I'd plug into shore power, the converter would buzz because of the heavy draw caused by those batteries as they charged up. Once they approached full charge, the buzz would fade.

So check to be sure it's your ATS buzzing and not your converter.