Active Fuel Management (cylinder deactivation) causes accelerated lifter wear on sustained highway driving at 55–75 mph. Symptoms: AFM tick or knock after warm-up. Very common failure at 80k–120k miles.
Warm engine fully, then listen at idle for a ticking/knocking sound. Any persistent tick = AFM lifter damage possible. Ask if AFM has been disabled.
Est. cost: $2,500–$5,000 lifter replacement + DOD delete
Source: GM AFM/DOD lifter failure pattern