Speaker grounded ?

Do you have a spare SB-300 sound board ? Could be one of the caps on the board, and swapping the board with a spare might isolate the issue.

Will check the grounding. Two working SB-300's were tested in game,both good but still the hum was there. Also these boards test good in a Quicksilver and no hum . Think it may be a Grounding issue i'll get him to check and i will be trying a Altek board on game.So i'll post on the outcome