I would like to see the circuit that actually slows down the fan without making a lot of smoke or noise. It seems that it would take an awful lot of filtering to remove the harmonics which are responsible for the additional noise, and if that were applied to the original circuit, wouldn't the currents flowing in the high Q resonant circuit prevent the triac from commutating off?
Anybody who has really solved this, please post here because I want to make one for my own use.