RFI can infiltrate into a Sonar system from several access entries ...

*Thru the power circuit ...
*Thru the xducer or its cabling ...
*Thru the Ethernet ...
*Thru the head unit itself ...

Remove the xducer from the TM and hang it in the water away from the TM ...

If the RFI goes away - it's infiltrating thru the xducer or its cabling ...

If the RFI is still present ... Then start searching for the intrusion thru the power circuit next ...

Rickie