I brought this up the the dealership mechanics during first (free) oil change and was told it was the nature of the beast. I once had a vehicle with a bad shifter fork out of the factory that would pop out of gear during engine braking. This seems innocuclous but annoying.
If you move the shifter very slightly, the vibrations will disappear. It appears it's a case of the cable transferring vibrations to the shifter. This is my first cable shifter, so I'm not sure how the shift lever is located (detents or whatever). Possibly a wee bit sloppy in shift lever detents?
Looking at the cable, it drapes over the top of the tranny, then makes a loop where it contacts the engine block, finally going down to the passenger side of the tranny where it attaches to the tranny. It also could be that some additional brackets or padding might isolate the vibrations. Currently, it kinda looks like the only actual attachmets are at the shifter and where the cable mates with the transmission.