In the early 60s, I lived about three miles from where the Rotodyne was being test flown at White Waltham. Running just along the northern boundary of the airfield is the GWR main line, where in those days, Kings and Castles were pounding along at 80-90 mph.
The tip-jet noise of the Rotodyne sounded
exactly like the noise of a King or Castle at speed.
There is a very good History Channel documentary in four parts on YouTube: