Bsshhhhhhhhhhhhhhhhhhhh thummmmmmmmmmmmmmmmmmm, zhuuuum, zhummmmmmmmm, thummmmmmmmmmmm, Khssssssssssssssssss (that's when it clashes with another lightsabre)
I read an interview with Ewan McGregor and in it he told how filming had to be restarted several times as he couldn't stop making the lightsabre noises when they were shotting the lightsabre duels!!