starter problems, I think?
 

I have a 1984 944 automatic, I just tried to start the car and got nothing but a humming sound when I turned it over. I tried to start it several times, but nothing. I gave it a few minutes and tried again and it started rite up. I just installed a new alternator and battery. I also noticed if I hold it in the on position or crank it over several times it will catch. Again, it doesn't make any noise but a humming sound when I turn it over. Bag ignition switch, perhaps, or bad starter? Thanks in advance for your help!!

My guess is that the armature in your starter is no longer engaging and the whirring you hear is the starter motor spinning without coupling correctly to the flywheel.

Concure. Clean the grounds at the chassis and at the engine.