Well got to route of the problem but not before lifting the engine and removing the shaft. I found a washer underneath the head of one of the bolts on the R & D coupling that shouldn't have been there. It meant the bolt coming into contact with the face of the coupling when the thrust went on, wear apparent on both surfaces. I could have sworn the noise was coming from far further back. Wish I could blame someone else, I had even checked the bolts for tightness several times and not spotted it.
