Reving up

I just purchased a 08 f350 6.4l bone stock with 53k in it. I test drive it for a while and put almost 75 miles on it during the test run. It ran flawlessly. I bought it and now it just sounds like its reving up going down the road. Nothing changes, the tach doesn't move, it doesn't seem to lose power, it just sounds like the motor is revving but it's not.

Wasn't sure how to search for this problem