[Elecraft] Heil headset with HC-4 revisited

[Bert]

The original Heil headset with the HC-4 element didn't work with the KX3.

I have now replaced the HC-4 with an electret, which sounded
great trying it with the KX3 before installing it in the Heil headset.
The electret, by itself, was plugged in to the KX3 with a stereo plug.

After electret installed in the headset I have quite a lot of hum. The
headset mic is connected with a mono plug to the KX3.

Any ideas why the hum appears after mounting the electret in the headset?

Bert VE3NR