Yeah, it's definitely not that. I pulled the whole thing off the bike and tested it. I can hear the smallest "click" from the sensor when I wave the magnet near it. Not sure what's in the sensor, but whatever variety of switch it is, it goes click when it's tripped. The fork just picks it up and amplifies

There might be no good solution here. Maybe other brands of computers don't have the same sound, like the Topeak you mention, because of a different kind of sensor mechanism? I'll have to ask some friends to let me stick my head next to the whirring spokes for a listen.

Thanks to all for quick replies.