Can you explain this more? Or link some stuff explaining more...would like to understand what you mean exactly.

can try, basically a motor-sensor loop can "hold position" by noticing when it has gone past some threshold then corrects in the opposite direction ... rinse and repeat.

Some systems may have thresholds that cause the rudder to "quiver" while staying on course, To an intelligent creature exquisitely sensitive to things quivering in the water this could be a major annoyance.

the main types of motors likely used in this application are Servos and Steppers

https://en.wikipedia.org/wiki/Servomotor

https://en.wikipedia.org/wiki/Stepper_motor

Here is a page that describes the behavior and calls it "hunting"

https://www.motioncontroltips.com/what-is-hunting-in-the-con...

But we are far afield from what I know, so apologies to the EEs and Robotasists for my gross simplifications.

That was what I thought you meant. And yeah that was pretty much exactly what I was thinking.

If the current was pushing against the rudder in waves it would also make a rhythmic pitch change that went along with the water movement.

If the whales hit the rudder it makes a "whine"...which is what an animal might do as well.