Lighting – Identifying a bulb from a 1969 built light fitting

Well Cabbey, you are not alone. I have run into the same problem with several fixtures designed to accept standard G25 type bulbs when trying to use CFL's. There is an obvious difference in the neck shape and dimensions. As far as the fixture is concerned, maybe you could go to Home Depot and take a look at the fixture you have at home and see if a simple modification is possible before you take yours down. I have also found a big difference in the performance of different brands of CFL's. I have purchased many "Light Effects" at Lowes with mixed satisfaction. Some work great, light fast etc, and some were very lazy. They have always taken back ones I was not happy with. I have had better results with Sylvania and GE brand CFL's, even using them outdoors in Maine.

What exactly is this sound?

It is called the 'mains hum'. It is usually emitted by transformers or arcs. The cause of this sound is that the electricity coming to your bulb is constantly changing direction/polarity 60 times (or 50 if you're outside the US) a second. The electricity is moving either the transformer if you're on a CFL or LED, or somewhere around an arc or loose connection if you're on an incandescent bulb.

tl;dr 1. It's the mains hum. 2. If you're on an incandescent bulb, you have to start worrying about it.