The short answer
A moving part is on its way out. Squealing is often a blower belt or a bearing, grinding is often a motor bearing or a fan blade, and rattling is often a loose panel or mount. It is usually a repair, and better done before the part lets go.
What you can check yourself
- Turn the unit off if it is grinding or screaming.
- Check for a loose panel, a branch or a toy in the outdoor unit.
- Note where the noise is: inside at the blower, or outside at the condenser.
What gets checked first
- Blower belt and bearings
- Condenser fan blade and motor
- Loose panels and mounts
Repair or replace?
Usually a repair, and better before it lets go.
When to call now
A grinding sound means stop the unit. Running it can turn a belt or a bearing into a motor.