my speakers make a high pitch constant noise. any solutions?

[unwashed potato!]

unfortunately for a few months now my surround sound system is making a high pitch constant noise, the sort of noise any higher pitched and only dogs would hear it.

i'm considering replacing the whole system as it's well over 10 years old now, however ocuk don't seem to sell anything under £200 and i can't justify that price.

i've tried unplugging each speaker from the base unit but the noise still comes so assume it's the base that's causing the noise. got me thinking is it possible to just buy a base unit (not sure the correct name for this) and plug the old speakers into that?

if so, any suggestions?

 

[unwashed potato!]

it's 5.1 speakers that connect to the sub, which connects then to both the sound card of the pc as well as chromecast audio. without anything playing and anything connected other than the speakers, the high pitch noise is there so can't be from whatever is sending the sound as i've unplugged them.