My wife got out of shower and asked me what the "buzzing" noise coming from the shower was. It sounded "electrical" so I started by throwing switches in the shower, dressing area, closet behind shower.
Then I went to master electrical panel and called her while she was right next to the noise in the wall. I threw one breaker at a time until I had turned all of them off and back on with no change.
Then, I went online looking for answers, most of which clearly were not the same issue. However, I did turn off the mail water valve, open ALL faucets (inside/outside). Close all faucets and then reopen main line.
I then took the shower control valve apart to no avail.
I then turned off the supply lines to the toilets (per an online post) with no luck.
I don't think it is "water hammer" since it is non-stop and is not a banging noise, but just a buzzing sound and localized to only one wall where shower control valve is located.
It sounds like a loud elect motor but there are none anywhere close to it.
I have been looking online for about 2 hours and tried anything that sounded remotely good.
Facts:
Dallas, TX area, slab foundation, single story 3/2/2, built in 70's, no changes to electrical or plumbing for many months.
Have noticed a sewer gas smell coming from shower drain in last few weeks.
The walk-in shower appears to be a conversion from a tub/shower to a walk-in with sliding doors.

OH NO!. I was reading the post to my wife about the people who left a razor on in the shower and we were having a good laugh when she jumped up and said "on no, I hope it isn't my battery operated face scrubber still on from my shower!"... IT WAS!
We are still laughing at ourselves and very thankful of the honesty of the other post which led to our solution. It had new batteries and would have run until the plumber showed up tomorrow on overtime to "fix" the problem.