I spent many happy days as a kid at the western end of 10R watching streams of BEA and other( LH KL EI ....) Viscounts whistle their cheeerful way to the Block 79?? holding point before lining up and bingo just as described here a clearly perceptible pause and drop in sound level as the whistle disappeared and was repalced by a deeper growl as the brakes came off and they disappeared towards the east. Very very distinctive and fascinating to read this thread describing possible reasons for it.
I only flew on the Viscount a couple of times-to Jersey I think but a really nice airplane from a pax view point and those windows wow!
PB