It started on a long highway trip a couple weeks back. It seemed to go away when I returned home, but it’s showing up at speeds above 70mph. It sounds exac5ly like there’s some small branches under the car, dragging on the pavement. But I don’t see anything.
I haven’t unscrewed the wheel well covers, or done anything beyond laying down and looking under the car. Any ideas?
I don’t know but have had a similar issue. Mine isn’t associated with speed but seems to be at rather lower speed and particularly after a turn like in a residential street intersection. Like you describe, it’s very much like the light thumps and dragging sound associated with a caught branch. My best guess is a plastic guard or other undercarriage item and tends to rotate clear out of the way but very occasionally rotates forward to make the sounds.
bump
Does it go away when you press on the brake? Could be brake wear indicators.
I’ve taken another look, with better lighting. It’s probably a plastic guard that’s broken and inadequately secured. I’ll put another bolt or zip tie in it, and see if that fixes things.
Thanx for the input.
The Parts Man in me is screaming, Year, Make, Model!
Prius 2010
I’d like to be perfectly clear: There is no Parts Man in me, now or ever.
As a previous posted indicated, press lightly on the brake pedal. If the sound goes away or lessens, then you’re likely in for a brake job as those will be the brake pad wear indicators telling you, your pads are worn out. Replace the pads before they’re down to the steel or you’ll need new brake rotors. Then you’re talking serious money.
Yes, Mr. Milhous. 
I think that’s a best bet given the described circumstance. At a high enough speed a piece of plastic shielding under the engine or in a wheel well is being pushed against the tire or road surface.
For a few months I was hearing a swishing sound coming from the rear passenger side. It was random, not associated with speed or braking. And sometimes it would sound like small sticks and rocks rolling around inside the body of the car. I did check around the back tire but saw nothing. I planned to take it to the shop after I was fully vaccinated.
I was driving last week and heard the sound. Then I saw a wire hanging down outside the passenger window. It was a windy day and it was scraping across both side windows. When I got home, I realized it had come loose from the groove above the doors. The rubber covering had deteriorated. I put it back in and stuck a piece of duct tape in the groove to hold it for now.
So it could be something as simple as that. It had been scraping the back window and body and I just hadn’t seen it. I had looked under the car, not on top of it.
Yeah but it’s still a 2010 Prius. 
If that doesn’t last long you may want to try tying it up with some 550 paracord. I love that stuff.
Before we sold my son’s Cobalt as a roadside beater, I swapped his relatively new tires and rims out for the well worn tires on my Saturn. They are a touch larger and wider. Now when I hit freeway speeds, the plastic rear quarter panels flex enough for the wheel well to rub against the tires. I found the marks after a few weeks of searching. After 20k miles now it seems to have gone away. It IS worse in summer so I will find out in a few months.