DIAGNOSIS That's the sound of the paper feed mechanism struggling. It's usually caused by the paper not being loaded correctly, or something (like a small scrap) stuck in the paper path.
FIX First, power off the printer. 1) Remove all paper from the rear feed tray and the cassette. 2) Open the rear cover and the front maintenance cover. 3) Shine a flashlight inside and look for any torn bits of paper, paper clips, or debris. If you see something, carefully pull it out in the direction the paper travels. 4) Reload your paper. Make sure the stack is under the max line, the guides are snug against the edges, and you've flattened any curls. Turn the printer back on.
IF THAT DIDN'T WORK If the noise persists with no debris, the paper might be too thick, glossy, or damp. Try a fresh ream of standard 20 lb. copy paper. If it still grinds, there could be a worn gear in the feed assembly, which would require a service call.
HEADS UP Always fan a new stack of paper before loading it to prevent sheets from sticking together. That's a common culprit for the printer trying to feed multiple pages at once, which causes that nasty grinding sound.