really....weird audio humming noise

Oct 12, 2018

I have a strange buzzing noise that sometimes comes from a laptop/subwoofer setup under certain conditions. laptop is a fujitsu lifebook a531, sub is this

it happens under the following conditions

hum is virtually silent at subwoofer bass dial set to minimum. hum gets louder as subwoofer bass dial is turned up to halfway, hum dies down to almost nothing as sub bass dial goes to maximum

hum is made worse when an hdmi cable (2 metres long) connected to a TV is plugged into the laptops hdmi port

my gut instinct is that its to do with the laptops internal earthing. educated guess. I've just ordered a new hdmi cable, which should get here in a couple of days. but the hdmi cable I'm using at the moment isn't a cheapy

any ideas welcome..? more info can be given

The sub is still under 30 day trial so I can send it back for a replacement