AC Compressor Knock / Metallic Sound — Liquid Slug / Mechanical Fault

Identify knock cause (liquid slug, overcharge, frozen coil, internal damage)

Check: Knock once or twice at start = normal. Continuous knock = liquid slug suspected. High suction + slug noise = overcharge or EEV stuck open. Internal grinding metal = bearing wear (end of life). Suction accumulator failure = frequent liquid slug.

What to do: On persistent knock, STOP system immediately. Inspect suction accumulator (prevents liquid slug). For liquid slug = recover, vacuum, recharge by weight. Bearing wear = replace compressor (hermetic, not repairable). Overcharge = recover excess. Failed accumulator = replace part.