After all the mechanical suggestions were tried without any significant improvement I decided to take a different approach. I Looked at problem from a "sound" and "initiate sound" point of view. Obtained Railsounds Power Board, Railsounds 4.0 / Generic Steam board and R2LC/Radio Receiver board.
After opening the tender and replacing the two sound cards one at a time, not finding any joy, I moved on to the engine and replaced the R2LC Radio Receiver board and everything was fun again. Engine back to normal no more intermittent whistle blowing. It cost me a few bucks but its nice to have this A5 PRR back to normal again.
My thanks to all those who replied.