After Maintenance New "Flapping" Sound

I just got some quick maintenance done today. I work weird and long hours so getting maintenance on my car is a hassle. I needed an oil change so I went to a Valvoline for an oil change. Not the best place for maintenance but it fits into my schedule best. Since I was there I figured I would get a few other things done that I had put off for a while. I had them change my radiator fluid and rotate my tires on top of the oil change. At 140k my Prius runs really well and I only do the necessary maintenance and upkeep. Oil changes at 10k intervals. I changed out my own transaxle fluid when I was visiting family (aka had an actual garage to use) back at 90k and haven't had any issues with anything since that last "milestone" maintenance.

Today after my oil change and other add-on services I drove around surface streets for a bit while running other errands. Nothing seemed out of the ordinary.

When I got into the freeway and after traffic cleared I was finally able to hit 50mph. But then my car started sounding LOUD. It isn't high pitched. It isn't a knocking sound. It sounds like a flapping/thumping sound. Almost like a flat tire or a loose wheel well liner but it isn't a flat tire or liner. It gets louder up until about 72mph and then it almost seems to quiet down.

I recorded the sound while driving and checked under the hood when I finally exited the freeway. It doesn't seem to affect the driving aspect and at this point seems to be mostly just annoying. I drive without radio often (I like the sound of driving) so I'm pretty used to my cars noises and this definitely seems off. Volume up all the way to hear best.

Any suggestions about what this sound could be would be greatly appreciated before I go back to Valvoline to tell them they messed up.