Actually it does have some. The rudder feedback unit effectivly controlls how fast and for how long the hydraulic pump runs for (it's a standard reversing pump, right? RPU 80, 160 or 300).
So with the rudder angle sensor connected the AP says for example, move to port. Pump starts but nothing happens, so AP says to pump "go flat out" still nothing happens = timeout.
Without rudder angle sensor, pump just runs for some time, and presumes helm
moves as required. So the loading applied - Amount of pressure and volume the hydraulics produce is different in each case.
Does the noise
seem to come from the pump unit or the ram?
If the pump has dodgy bearings, this could happen... Although if the pump is really bad it will likely overload the AP computer, which will fail with a Current
Overload type message.
Which AP computer do you have - the J300X ? if so it can only supply 10A...and the pump (RPU80) can use up to about 40ams at stall...
Anyway, if you can tell me if the noise
is from the pump or ram, we can probably fault find this a bit better!