all of the blue arching you see at the brushes when it is running on AC creates heat and noise. when you switch to DC the blue arch goes away. the fan also causes noise but the high scream comes from the arching..
you can also machine 1/2 the the fan away when you have it apart to help reduce noise.
I looked into building your converter yesterday and other than the bridge rectifier the parts were fairly expensive. I had no clue that a rotary dial dimmer switch could cost from 80 to over 250.00. Do you have any recommendations where to get said parts a little more reasonably? I would hate to spend a couple of hundred dollars on an experiment.