There are only a couple places a sound like that could originate. When you put it in L, the only thing that really changes is the aggressiveness of the regen braking - there are no other physical gears to select.
That suggests some load dependent vibration in the drivetrain. I don't know how concerning it should be but it is absolutely not normal.
One other possibility is something called "camming" (If I'm remembering that correctly) that can occur with synchronous AC motors like those used in the Focus. They rely on a controller to properly time the triggering of each input phase, having the coil "behind" pushing while the coil in front is pulling. This relies on different types of feedback to know if it's doing it correctly. Sometimes, the controller can end up out of sync enough for those pushes to be misaligned which causes the driveshaft to experience pushes and pulls aggressively shifting back and forth rather than smooth mostly continuous force like it should.
For a while my 2012 was doing what I've described if I got in and started driving too quickly. My only guess is that the green "start now" icon was not taking into account the actual motor controller being fully started up. When it would happen, shutting the car down completely and turning it back on less aggressively would stop it. It eventually stopped happening at all - I think due to a TCM software update applied by the dealer. They never caught it in the act, so if yours is doing it all the time I'd suggest getting it looked at.
2012 Candy Blue FFE
33.5kWh battery pack
CHAdeMO DC Fast Charging