Grinding noise after driving through water

Lots of flooded roads yesterday, and I drove through a few deep water spots – maybe 8"-10" mostly at VERY slow speeds. The car is a 2009 Toyota Matrix, standard transmission. It was pouring yesterday, and very loud so I don’t know if the grinding noise started yesterday. Today when I first started it, I heard a grating/grinding noise in the front about in the middle. It sounds a bit like I’m dragging a large stick or a piece of metal under the car, but there is nothing dragging. I can be coasting, sitting still (but only when I first start it), accelerating, or braking. That said, it’s not constant. It comes and goes as I’m accelerating. Once I get up to speed in 4th, I don’t hear it.
Thoughts on what it might be? Usually, I just hope that noises will go away. Do I need a mechanic to check it out?

Could be a small pebble or a bunch of grit/debris has gotten stuck in the wheel or brakes. Try to figure out which corner the noise seems to be coming from, then ask your mechanic to remove the wheel and do a visual inspection.

that’s what I thought at first, but I do hear it when I first start the car, before I am moving.

Check the fender liner on the passenger side. It may have broken loose and is making contact with the pulleys and belts.

Before starting the engine again you may want to open the hood and take a good look-see. It’s possible that floating debris like a stick got into you engine compartment. The height of the water you described would be just about right for that to happen, and floodwaters are full of that type of debris.

thanks for your suggestions. It seems like it might be the heat shield, which appears to be a little loose.

If not loose, maybe the converter heat shield has developed a crack. The converter runs near red hot and immersing in cold water suddenly could cause a fracture. The tiniest fracture can cause a buzzing that can easily come across as a grinding noise.

Good thought. It might have even caused one of the heat shield’s tack welds to pop.
Could have fractured the ceramic substrate too.