Over time 964s develop a lot of little quirks, one of which is a buzzing noise from the dash board. The buzzing starts as soon as you turn the key and carries on for 20 minutes even after the ignition is switched off. This is caused by an air sampling fan at the back of the climate control unit. So today I took out the unit (comes out like a radio) and took apart the fan.
After removing nearly 20 years of dust and fluff, the little fan know runs quietly and the buzzing is gone.