1198 si – engine noise on sonar over 1k rpms

help

helluva first post !!
several things can cause this regardless of brand of fish finder or motor.
poor connections at battery first and foremost.
wires running parallel with wires to battery,non resistor spark plugs in motor.
rectifier going south on you,stator resistance high,loose magnets in flywheel,poor ground at either the motor or fish finder.

I have found lately that an older battery regardless of brand uses a cheap paste at the bottom of the lead plates,when this paste gets saturated with battery acid the resistances goes higher which places a large load on the charging coils causing them to heat up as the demand on them is higher due to higher resistance in the battery,when this happens electrical noise is greater also.

I recommend for the moment that you give the finder its own power source even if it means a second battery,if that cures it,you can start hooking up other items from the main battery until the noise appears.
this could be as simple as a livewell pump,ignition noise,( plugs,trigger,stator,etc.)

once you have isolated what is causing the problem report back and we will take it from there.