My 66 Impala SS makes a pretty hard "clunk" when it shifts down from 2. to 1. gear. It doesn't make a differance if I drive / brake hard or if I just let go of the gas and let it slow down untill it shifts at appr. 15 mph. Sometimes it shifts smooth, but most of the time it is pretty hard and it makes a loud clank. I just put in a new vacuum modulator, and I think I am more aware of ugly sounds now, but I know I have noticed this before. Can anyone tell me what causes it? I suppose the adjusting screw on the modulator goes only for the shiftpoint-speed?