2017 Prius "grinding" noise near end of two 30-mile drives

Only my wife has experienced hearing what she calls a "grinding" noise after driving 25 of 30 miles on two trips. Once it starts, it doesn't stop. After each visit, there was no noise upon driving away. But after the first visit, same thing. After the second visit, I told her not to drive the car so I could see/hear it. I drove the car for 30 miles. No noise. Drove it last night for 10 miles. No noise. About to drive it 30 miles right now. Will report back.

Toyota told me a year ago to replace the rotors because of rust. I didn't. I did the shift -to-N and coast down a hill routine. Got a second opinion from reputable non -Toyota guy. No problem with the rust, he said.

Just had service. No mention of brake issues.

Any thoughts?

My brother-in-law and I think it's brake noise, but why not constant?

(One thought that I doubt applies: I think it was our 1984 Accord that had brake warning noise, a high-pitched grind like metal on a lathe, where little prongs of metal would rub against some moving part when pads were worn enough. When noise first started, it would be intermittent, depending on terrain and driver. Thirty-year-old technology. Seemed a good idea. Probably not here.)

 

As promised I'm reporting back. My 30-mile drive was silent. Wife took 20-mile with more than usual grinding.

So I drove the '06 the same 20 miles. Got in the '17. Put it in reverse and immediately heard the noise for the first time. And "grinding" describes it perfectly.

I immediately said it's dragging something and decided to go to first place I might find with a lift. Backed out of the driveway and could pinpoint noise to the left rear. After driving 100 yards, I remembered seeing something sticking out from the wheel well maybe a year ago. A six-inch piece of metal had skewered the plastic(?) and I pulled it out, glad that it missed the sidewall, and thought no more about it. So I got out but saw nothing. I felt around the very same area that had gotten itself impaled and found a loose piece of plastic(?). I couldn't pull it off to what it was no longer so firmly attached to, but I was able to rotate it (I guess around a screw) so that a shorter protrusion was nowhere near the tire that was indeed grinding the longer protrusion that I had located 180° from its previous position.

Problem solved. Temporarily? Maybe. When wife brings it home I might see if I can remove that piece permanently. If not, we know the noise and how to deal with it.

Whew.

 

The piece is sort of rectangular, about 2" in width and 4" in total length with a screw located 1" from one end and 3" from the other. The noise resulted from the piece being situated so that the 3" part was intermittently rubbing the tire. Vibration caused the piece to more or less randomly rotate about the screw. When positioned so that the 3" piece was pointing within about 10 degrees of a line drawn directly at the tire, we had noise. When the piece had rotated more than 10 degrees from that line there was no noise.

The piece was sheared off from the wheel well shroud and is no longer connected to anything except the screw. I rotated it so that the 3" piece was pointing toward the front of the car and the 1" piece was pointing at the tire and at least 2" from the tire. So I don't think it could "wind itself up around the tire". Vibration would have to cause the piece to rotate 180 degrees in order to cause noise again. That seems unlikely. But if the noise does recur, we know what to do.