DIAGNOSIS That grinding or rattling is almost always air in the pump line or a dry pump. The internal condensate pump isn't designed to run dry, and if the water path from the tank to the pump has an air bubble, it'll sound terrible.
FIX Unplug the unit. Disconnect the clear pump hose from the back of the dehumidifier (where it connects to pump out). Hold the end of that hose in a bucket or sink lower than the unit. Use a small funnel to slowly pour about a cup of water back into
IF THAT DIDN'T WORK If the noise continues, the pump impeller itself might have a bit of mineral scale or debris jammed in it. You'd need to access the pump housing (usually a few screws on the bottom) to check and clear it, but that's a more involved fix.
HEADS UP Always ensure the drain hose runs downhill without any loops or high points where air can get trapped. A consistently primed line is key to a quiet pump.