My 2015 Silverado just started doing this today, at first it was a quick rumble, like I drove over something. 35-45mph zones where it shifts a lot. RPM fluctuates 100-200rpm. I plan on doing a full trans service, dropping pan, possibly tomorrow. Hopefully it will clean out the trans, torque converter etc. If I have to I'll replace the tC and transmission. We are just about to turn 203,000 miles. so far this has been our only issue with this truck.