Are you sure it isn't coming from behind the interior light?
100%.
You can feel it buzzing if you put your hand underneath where the cup holders are, short of removing the radio and ripping the dash out I am at a loss. Obviously it isn't the fan as it is not a rotating noise at all and is there all the time even on the first turn of the key, but I didn't know if they had some sort of ambient flow fan or something which kept air circulating?
I don't want to go ripping the dash out, because my 172 doesn't creek or rattle AT ALL and I want it to stay that way. Removing interior plastics and I do not get on, they like patience...I have none.
Just wondered if it was a common or known problem, if not I guess I'll leave it and see if anything breaks.