Diagnosis
That's almost certainly the fuser unit—the hot rollers that melt toner onto the paper. A grinding sound usually means a worn gear or bearing inside it.
How to Fix It
You'll need to replace the fuser assembly (part number FM-350). Power off and unplug the printer. Open the rear cover (the one on the back, not the front). You'll see a green handle. Lift it to release the fuser, then slide the whole unit straight out. Slide the new FM-350 in, lower the handle, and close the cover.
If That Didn't Work
If the noise seems more like a high-pitched squeal or comes from the front, it could be a pickup roller in the paper tray struggling. Try cleaning the rubber pickup rollers with a lint-free cloth dampened slightly with water.
Heads Up
Let the printer cool down for 30 minutes before touching the fuser. It gets extremely hot during operation. Also, don't try to lubricate or repair the old fuser unit—it's a sealed, consumable part.