Paradism - I assure you that this is not about the Doppler effect. What I (i.e. the lad at perimeter fence all those years ago) am talking about is the spool-up behaviour at the end of the runway before brakes-off. Looking at my first post - I guess I could have made this clearer. This is all about stationary aircraft.
The response so far have confirmed my suspicion that it is to do with rate of change power demand. I think the subtle change in note as the water/meth comes in is not the answer we are looking for - although I'm sure it is audible at close quarters.
The only remaining question is why things (apparently) changed in the 80's.
One thing occurs to me: were the 70's Viscount pilots those who had progressed from pistons (Argonauts in BMA's case) and tended to treat the power-levers in a different way to the 80's pilots who might have had different previous experience ? ...