Brake pedal Pulses when stopped while in Drive or Reverse. Not when in Park

I finally figured it out today.

I spent so long looking at what I thought would be obvious root causes and found that the previous owner mounted an oil catch can to the side of the engine bay and it ever so slightly rubbed on the firewall. At lower RPM it vibrated and was transmitting to the break pedal. I moved the catch can and properly secured it. All good now.