Your troubleshooting steps are classic to the way interference should be tracked down ...
Step 1. Verify if the interference is intruding in thru xducer or its cable by separating them from the TM and test ... Your results of continued interference concluded that the xducer and/or its cable were not culpable ...
Step 2. Verify if the interference is intruding in thru the power circuit by testing possible fixes in the power circuit ... Your results of eliminated interference concluded that a ferrite in the power wire arrested your interference ...
Great job cray ...
Rickie