Hum in class D Sure Amplifier

I have a Sure 2 X 100Watt Dual Channel Class D HiFi Power Audio Amplifier Board - AA-AB32971 that I built as an amplifier for my computer.

I was having trouble with hum especially when the LED lights in my laundry room were turned on, or the wife ran a vacuum cleaner in the back room.

Also sometimes for no apparent reason I would just get hum at anything above low volume settings.

I tried line conditioners, filters, different outlets, ups's, audio isolation transformers, different connectors, and ferrite loops. Nothing would stop or reduce the hum. The audio isolation transformer would inject a local AM station directly in to the amp.

The power supply was a switching supply taken from an old laptop.

what fixed it was installing a 3 prong to 2 prong adapter on the plug. The lack of ground on the plug caused the problem to stop.