further investigation shows that the buzzing interference is only heard when something causes the screen picture to move/change. ie up or down scrolling but not sideways arrow keys that don't cause a change in the view.
Keyboard is wired, mouse is wireless, not sure what frequency. If unplugged at the sound output jack the noise goes away showing the amplifier to be quiet. I'll try a different sound source to see if the amp has gone a bit harsh but I doubt it is part of the problem.
Rans6.....