I'd try rotating the pot to see if there's dirt in there first as well, but if you turn the knob down slightly and the unusual noise stays constant without you moving the knob, it's also possible that you're suffering from a dirty/high impedence connection. It's sometimes caused by a cold solder joint (which exhibits high resistance), but in the case of an Alembic, it's more likely to be be oxidation on the molex connectors between the circuit board and the pickups, etc.
It can be a little tricky to fix this. The most effective way is probably to get a
Caig Labs DeOxit pen. The pen has a red contact cleaning liquid in it which is very effective, but what really makes this useful is that it has a sort of fiber tip which is saturated with the liquid after you prime the pen. The combination of the contact cleaner and the fiber tip really helps clean things up to a much greater extent than a Q-Tip or other swab. You unplug the connectors to the PC board, then wipe down the pins firmly with the pen. This doesn't directly solve the problem if the oxidation is on the plug end (pretty much inaccessible without replacing the plug), but I won't tell anybody if you push the plug on while the contact cleaner is still wet. The Molex plugs are also self-wiping, that is, they will clean themselves up if you connect and reconnect repeatedly, but they are NOT rated for hundreds of repetitions, so this isn't the best way to do this.
All that it takes is for somebody to handle the pins with their bare hands to contaminate the contacts before they're mated. The oxidation that results can increase the resistance of the connection, which can sometimes cause oscillations and electrical noise as you describe.
The humcanceller is the coil of one of the pickups without the magnets, so it won't make any noise if you tap it with a metallic object.
The best way to test if it's working is to measure it's resistance with an ohmmeter. I don't know what the proper value will be, but it should be very close to the values of the pickup coils as well.
I hope this helps,
David Fung
