DC-3 high pitch whine before engine startup
When I was a kid I flew frequently on well equipped 'corporate' DC-3s.
Just before starting up #2 I would always here a series hi pitch whines and buzzing coming from what appeared to be the equipment rack located on RHS immediately infront of the cabin/cockpit bulkhead.
I presume these were gyros comming to life in sequence after CBs were made or power supply dc-dc converter vibrators (although I have wondered if engine starter motros were responsible for the whine).
Any other suggestions?
I don't suppose all DC-3s make this noise as most are just flying tin pots poorly kitted out with a bare minimum of avionics.