Hi MO,
As people have said already, you need to get clarification from the engineers regarding the metal.... I am sure you already know this.
I would be confident to say that the control feedback has little to do with the metal found in the engine, it is most likely a 'happy' coincidense.
I think TET may have mentioned the sprag mounts already. I am pretty sure that they are the culprit for introduceing an oscillation. These are the four rubber mounts under the engine that connect the dry sump to the frame. when these rubbers change composition over time (which they always do) we get some pretty impressive changes to the overall feel of the aircraft. Remember, any movement at this attachment point will be directly transmitted to the rotor head as all the components are connected and pivot around the basket mounts. A move of around 10mm at the bottom will be exaggerated by a factor of 4 by the time it reaches the head.
If the lateral sprags find a frequency that is unstable then we will feel the aircraft 'shunt'. Feels like the machine has just been kicked in the rear... a very disconcerting change but easily recognised. If the longtitudinal sprags are stuffed then we get a lateral bounce in the machine. An interesting one is when either both sets play up or the resonance of one set interferes with the other...... we then get all sorts of wobbles..... This is what I think you are experinceing.
Get them checked while you are there. If you are out in the field, engineers can just change the torque on the bolts holding the sprags. This in turn changes the resonance of the sprags allowing you to see which ones are playing up.
Anyway, I am sure you are going to follow through with the engine prob so we look forward to hearing what has happened to the old girl. I would be interested to hear if it has anything to do with the control feed back.......
Last thing, the 47 and KH4's are a wonderful aircraft. Sure they have problems as any older airframe is expected too, but not once in my career operating them did they not 'tell' me there was something wrong..... It sounds like you have listened to yours as well MO... good one.
Cheers