Safety first: Turn off power before tightening screws or removing covers.

Check mounting: Gently push on the indoor unit. If it rocks, tighten the mounting plate screws into the wall anchors.
Inspect blower: Remove the front panel and filters. Look for loose objects or broken plastic in the blower area.
Tighten screws: Tighten visible screws on the indoor casing and fan motor bracket.
Add padding: If plastic panels vibrate, apply thin foam tape between contacting surfaces to dampen noise.

Loud rattling or buzzing noise from the indoor unit during operation.

Possible Causes