I have a 2015 Suburban LT 6 speed transmission. About a week ago I was on the expressway and notice an occasional rumble at 75mph while in drive. It has gotten worse since then. When in 5th gear it does not happen. The tranny seems to be making more noise than usual but that could be just imagined. Hard shifting from 2nd to 3rd also. Is it the AFM?

 

It very easily could be the AFM or the transmission. Care to share some info since you provided none. Miles, last trans service, any recent work done, condition of trans fluid.....etc.

 

I've got 173,550 on it. No recent work. In checking the transmission fluid from the dip stick it is slightly dirty but no burn smell and it's at the correct level. I bought it at 159,000 and I believe they did the proper maintenance on it - changed fluid and filter at 135,000 miles. Of course I talked to a transmission shop and they're just parts changers not mechanics or know any symptomatic problem fixes other than it's a bad transmission.

 

You have almost 40k miles since fluid and filter change. Do a trans service and replace the filter. Maintenance is never supposed to fix anything but in this case it might. Post a pic of the "slightly dirty" fluid. Plus, when you say you "believe they did the proper maintenance on it" what does that mean? Paperwork proof is something I can get behind but guesswork, is just that, a guess and a hope. You have 173,550 miles on a 5 year old vehicle. That is an average of 34,700 miles a year. That is a bunch of miles on that new of a vehicle. That trans may be ready to give up the ghost at that mileage, especially if too many miles between services. Your "parts changer" transmission guys may be right on the money.