Diagnosis
The noise is likely from the condensate pump. A failing pump motor or a foreign object caught in the impeller causes grinding. A loose pump mounting causes rattling.
How to Fix It
Unplug the unit. Access the pump compartment, typically behind a lower rear panel. Check that the pump is securely mounted; tighten any mounting screws. Inspect the pump inlet screen for debris. Gently shake the pump; if you hear loose parts inside, the impeller is damaged.
If That Didn't Work
The fan blade may be hitting the housing or a wire. Remove the rear grille (unplugged) and visually inspect the fan for obstructions or cracks. Manually spin the fan to check for wobble.
Heads Up
A low hum from the pump is normal during its 2.5-minute cycle. The grinding/rattling is distinct and indicates mechanical failure requiring pump replacement.