squealing Plantronic mic

Mike problems could be caused by the antenna as Bob suggests, or it could be
caused by a ground loop.

A ground loop in the mike circuit might occur if there is more than one
grounding point - for example where the shielded mike cable is grounded at
the radio and also grounded at the metal panel where the mike connector is
mounted.

Sometimes simply insulating the mike connector from the panel will cure the
problem.

Colin