[Elecraft] Audio Hum in TX Data Mode

[Conrad PA5Y]

I put up a new 6m antenna mainly for EME and I was so looking forward to using my K3S. The RX is great, I run it in parallel with 2 other receivers and it works very well on EME. However try as I might I cannot get rid of some quite bad hum on the TX Audio. I am using USB and before you ask yes I have got a low impedance ground from the PC to the K3S although removing it makes absolutely no difference.  What did make a difference was moving the radio away from sources of magnetic fields such as monitors. However I cannot get rid of it completely. It also increases or at least changes when I put my hands on the radio. 

I have a few other radios located nearby and close to a monitor, pc or PSU and they do not suffer from this at all. Is this a common problem, is it a bad ground in the audio path internally or magnetic coupling into an audio transformer?

It is completely impractical for me to move my radios 10 feet from any magnetic fields. Short of wrapping the radio in mu metal I really do not seem to be able to fix this.

Any ideas? I have read and understood and followed Jim K9YC's advice and this works perfectly for my other 3 radios. I am really at a loss.

73

Conrad PA5Y