I have the same Rumble strip noise coming from my 2014 Silverado, 6 speed. The rumble strip noise seems to be random as well, and at times I noticed the tach increasing in RPM's, by only 100 or 200 RPM during this time. I then decided to hit the gas the moment the rumbling began and it stopped immediately. So then I drove on the highway using manual shift for about 45 minutes and never encountered the rumble strip sound and vibration, other than actual defects in the road surface. So I'm not certain what's causing the issue.
Also, It does shift hard during moderate to slow acceleration from 1st to 2nd and sometimes from 2nd to 3rd, but if you accelerate quickly (not hammering it) from a stop the shift is fine.
Any advice from you folks?