I think I would describe the sound as 'screaming' ... not the normal sort of 'hissing' sound. I'll experiment tomorrow and do a vid...I bought a heap of cutters from toolstation yesterday, 3 each of 6mm, 1/4", 8mm and 10mm so plenty to experiment with though I'd prefer not to break them all! Before I made the recent changes to the router I can't remember ever breaking a cutter ... it was just too slow. I can use much more than 2.5m/min if neccecary - I tried a 1/2" cutter yesterday cutting 1/2" deep and it was fine at 6m/min.