I had this issue with my truck I have a 2015 Chevy Silverado 1500 Crew Cab LTZ Z71 with the 5.3l and 3.42 rear end and 6800 miles. I would notice more of a humming when I was cruising down the highway. It had nothing to do with MPH more RPM whenever my truck would settle in at the 1750 RPM I would have a humming that I could feel in the steering wheel and pedal. I took it to my previous dealer and they said I was crazy then I took it to a second dealer and they said they have had about 3-4 complaints about this. They told me that GM knows about this issue and say it has to do with the harmonics in the exhaust system causing a vibration and they are developing and new exhaust for the Silverados. I asked when this fix was going to be available and they said they have NO CLUE they are designing a new exhaust. I was lucky enough to get a dealer to work with me and are giving me a great trade in on my truck so I just ordered a 2016 LTZ Z71 all the loaners I had have been new 2016 trucks and did not have the issue.
