About a year ago the steering wheel was pulled to change the evaporator coil. The sound started intermittently then. It worsened with time. I think they left something in the assembly lose.
I can’t find it, but there is a thread on here about the steering wheel nut prematurely failing (and I believe there is a Ford TSB on it too)... not sure if there is an actual recall for the Mustang, but there should be one as it can be a safety issue.
Some folks have bought a new nut to fix the problem.
I wonder if they tightened it back under the hood? Also, the steering wheel has a bolt that holds it on. Under the dash is a rubber flex joint that let's the SW be tilted. I'd lay under the dash and car and get someone to play with the SW and see what's going on.
Problem fixed. Wasn't the U-joint to the rack or the boot on the floor board. All that was good. When the evap coil was changed a little over a year ago the tech didn't tighten a bolt connecting the collapsible part to the steering wheel under the dash. The bolt was half way out. All is tight and quiet and it drives great. Thank you for the help. Especially Cobra Jet's link. It gave me a good starting point.