aviation vhf handheld radio problem

I bought a Vertex VXA-210 vhf aviation handheld and am experiencing a
strange tone when attempting to transmit with my David Clark headset plugged
into it. The tone is not present if I disconnect the mic plug from the
headset adapter. The tone also fades away if I remove the "rubber ducky"
antenna or if I cover the mid-section of the antenna with my hand while
transmitting. The radio has a feature to turn off its built-in mic with the
headset connected and I have selected it to "off", but find it odd that when
the headset mic plug is disconnected and transmit, I can tap on the radio's
mic and faintly hear the tapping sound in my headset...

Any ideas?

Thanks,
Monty