the isv only gets power from the ECU, so it sounds like an electrics problem at the fusebox or with the ECU relay. Pull the ECU relay (see my fusebox FAQ for which one) If the ISV stops buzzing either the relay is stuck on, or the power to the fusebox terminals is goosed somehow.
Check pin 3 of the ecu relay socket, this is the ignition live feed for the relay, if it has 12v with the ignition off then you have a fusebox problem, if not its probably a buggered relay.
If that terminal is live, eityher there is an internal short in the gearbox, or the ignition switch is at fault. To test this theory, remove the lower ignition shoud trim from the steering column and locate the ignition switch plug. Look for the black wire, now without unplugging it you need to check if this has 12v or not with the ignition off, if you have a decent multimeter you should just about be able to manage to fit one opf the probes in to touch the contact. The other thing you can do is drop the fusebox down, and follow this wire to the other end where it plugs into the rear of the fusebox, and test it here
Obviously if its been spliced into at any point, investigate these wires to see if someone has done a shocking bodge job, or if its hooked to a faulty alarm unit which is energising the wire.
Let me know how you get one
