Could be a bad wheel bearing. I was having the same issue at 65-67mph. Dealer did some troubleshooting and rear passenger wheel bearing was bad. It was replaced under warranty ( 2014 DCSB w/24500 miles ). Quiet as can be now.
I would take it before the warranty is up. If it's bad they should replace it free of charge. It was definitely aggravating every time I crossed into the 65-67mph range. Now it's super quiet! I guess some will try to fool you saying it's tire issues, but a good dealer should make it right!
A guy I work with has 350,000 on his 05' DCSB. I wouldn't have believed it until I saw the odometer w/my own eyes. He has never had the engine or tranny rebuilt. Just replaced a wheel bearing and engine oil changes. He is just starting to have tranny issues w/it not shifting into overdrive but...
Update: The balance did not work! Still was getting the floorboard vibration and pulsing humming noise. Loudest at 65-67mph. Dealer did some troubleshooting ~ 4hrs worth and it turned out to be a bad passenger side rear wheel bearing. They replaced it and now it's quiet like it was when it...