I'm not 100% sure. I'm thinking it's caused by the fluid coursing through the lines or box, causing slight vibrations that are being passed up through the Bergman coupler and into the column, which is then heard inside the car. There's a video of a 68 Barracuda making the same noise. Here's the link:
Wow, that's quite worst than I thought. I'm not sure what all would cause that but my Borgeson box with a all-metal u-joint to the column doesn't make that noise.
