I think more or less every plane with any significant avionics does have an avionics cooling fan (behind the dash) and that is quite noisy.
The rest of the noise, as stated, comes from the gyroscopic instruments i.e. the horizon and turn coordinator.
This is why, if you leave the master switch ON and forget about it, even if everything else is turned off, when you get back after a few hours down the pub, the battery is flat
