It could be the fan – though I wouldn’t describe the fan noise as a buzzing noise – it’s most definitely a loud fan noise. My SMax has always done this and it is a definite high pitched electronic buzzing noise that is on for about 30 seconds then goes off then comes back on again intermittently with a crisp relay click sound when it switches on and off. I asked about this alot when the car was new (to me) and the dealership, as usual, were about as much use as a slap in the face with a wet fish. I have done frantic googling and querying other self-proclaimed specialists, but still not got a satisfactory answer or explanation. What I have managed to establish is that it is not a fault, is normal, has done it since i’ve had the car (now 6 years old) so i’ve just learned to ignored it.
Various suggestions i’ve had are that it is the climate control vent servo’s closing the climate control vents. This is BS as the noise is intermittent and goes on for some time after the car is switched off. If it were the climate control vents it would do it once after engine shut down then not again.
It could be a small pump in the fuel system that manages the very high fuel pressure once the engine is shut off by circulating the fuel around the system to prevent coking of fuel in the hot parts of the system and to maintain fuel pressure while the engine is off so that it starts when you come back to the car to start it – this sounds a bit more plausible as it does happen for a time after the car is switched off and then stops occurring, maybe about an hour or so after shut down, so could happen so long as the fuel in the manifold is at an elevated temperature and stops once it’s cooled.
Anyway whatever it is it’s not been a problem even if i’ve left the car on the drive for 2 weeks or so, so doesn’t run down the battery or anything.