This isn't something that is common or seen over and over. You're going to need a live person who can look underneath, but I can offer some suggestions. I had a problem a few years ago with my minivan that could fit your description perfectly. It occurred only when turning to the left. The engine would shift to the right a little, which is normal, but a metal plate protecting the fuel lines had rusted and shifted just enough that one of the pulleys on the engine was only 1/16" away from it. The engine shifting caused the pulley to rub on the bracket.
Engines also rock forward and backward when you accelerate and coast. The torque is greatest at low speeds, and that is when the engine will rock the most. By the time you hit 25 - 30 mph, the engine is starting to relax on its rubber mounts, so anything that was hitting or rubbing will stop. It doesn't have to be moving or rotating parts either. Exhaust noise will transmit into the car body and sound like grinding when a part of the exhaust system touches a metal part of the car body. The exhaust system hangs on rubber isolators, but if one of those is deteriorated, the two metal parts of the hanger can touch.
Corroded / loose heat shields under the floor of the car can also make noise. What they sound like can change as they get hot and expand. Usually that's more of rattle, but if they vibrate, they can sound like scraping too.
Wednesday, September 21st, 2016 AT 2:18 AM