I've built a couple of the 12Vac but never had a buzzing problem. Does it buzz in both channels?
Try removing the wires from the volume pot (or input jacks) and solder short jumpers across input connections on the board.
Try connecting the negative side of C11 to the chassis ground. The chassis ground should also be connected to the frame of the volume pot and the ground connection on the AC inlet.
Check to make sure all of the diodes and filter caps have been installed the right way round.
See if your build resembles the one in the link below. There are other parts to this build. Look around the WoS.ca site.
dc elevating the filaments cures the buzz, worked for me all the time..
Reflow all the solder joints. Check your ground connections. Measure voltages.
If it is just a faint hiss, shouldn't be a problem. If it actually sounds like a tone, could be a problem for your tweeters and power amp. Is the noise the same in both channels?