I think the device you're referring to is the idle speed controller. At
least in my 850 there is a buzzing noise when the ignition is on, and a
few seconds after it is switched off, so I figure it's normal. I'm not
sure if it comes from the controller though.

The camshaft position sensor is located right below the upper engine
mount that can be seen above the shiny thick S-shaped hose right of the
"20 VALVE" text in the picture. (You probably don't have that hose in
your car, but the engine mount is there.) The sensor is a disc about 5
centimetres across, fastened by two screws. If it has a shiny design
instead of a black plastic cover, it has probably already been replaced
at some point.

The control system uses the camshaft sensor to determine whether a given
cylinder is at the compression or exhaust stroke. A missing signal will
stop the engine and prevent restarting. I had a failed sensor some time
back and it caused exactly the symptoms you've described.

The crankshaft sensor, in turn, is used by the control system to
determine the speed of the engine. I've had this one fail as well - it
would first cause very short losses of power, then make the engine stall
but NOT prevent an immediate restart.

As I said before, the sensors should give a trouble code when faulty. If
there is a recurring TPS code, I'd check that sensor first.

Jari