This problem's come up before, and I think I solved it. Under your Prius, you have "guards" made of plastic and held in place by fasteners that unscrew off but press onto the bolt studs coming down from the metal. They basically shield the underside from debris but have holes to let water flow back out. I had a persistent squeak for the last couple of weeks that was really annoying me. So, I set out to see what I could do about it. I thought I'd have to add an adhesive pad as a "bumper" between two parts, but when I tapped on the guard (reproducing the noise), I could tell there was a lot of pea gravel in there. It took a while to get the first fastener off. They don't just "unscrew" that easily. The slightest pressure and you wind up pushing them back on. So, I had to work with two tools with one being a screwdriver gently prying it away from the panel so each turn made it come farther off the bolt. Once the first one was done, the rest were easy. Three in total on each side. I got a cereal bowl full of pea gravel out of each guard. I was shocked that so much road debris was trapped up on there, but this stuff was big enough to not flow out with the water. So, if you have an annoying squeak under your car, check to see if these shields have filled with debris. Service might require jacking up the side you're working on so you have room to get in there with both hands.
