For the past few weeks I've been hearing a clicking sound from somewhere in my dash, assuming it was the lifters or something in the engine bay. I spent most of my time in that area, looking around, and it wasn't until I tested out the steering wheel from our BMW 325i E30 race car that I noticed it got louder.
After a little bit of poking around the noise ended up coming from something I didn't expect.
I measured the hole and it appears to run straight down to the joint on the steering column, about 28 inches deep. My best guess (and certainly not scientific, so weigh in below if you have thoughts) is that the sound is originating from the engine bay and being amplified through the column to the point that it's essentially using the hole as a shotgun-pattern speaker right in your face. With my old Mtech 2 steering wheel installed the sound echoed back into the cluster and appeared to be coming from the dash itself.
To fix it we simply used some Ultra Black RTV and it quieted the cabin right up. The exact BMW parts that are causing the noise are still a mystery to me, so if you happen to have any clues be sure to leave them in the comments below.