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A heavy metallic knock that appears to come from the lower engine/bellhousing area and does not change during a cylinder-kill test points to a noise source that is not caused by cylinder firing pressure. A common light-repair cause is loose flexplate or torque converter bolts, which can produce a consistent knock as the flexplate rotates. A worn connecting-rod bearing often changes when its cylinder is disabled because removing combustion load reduces the force on that rod journal. Valve train noise is typically heard from the top of the engine and is more of a tick/tap than a deep knock. Spark knock generally occurs under load and is combustion-related, so it would not typically present as a steady knock at idle that is unaffected by cylinder kill.

Worn or sagging coil springs reduce the suspension’s ability to support vehicle weight at the designed ride height. When spring free height or spring rate is reduced, the suspension sits lower and has less available compression travel, so it reaches the bump stops more easily during normal driving. A faulty sway bar mainly affects body roll and side-to-side balance in turns rather than static ride height or straight-up-and-down suspension travel. Damaged shock absorbers (or struts) primarily control oscillation; they can worsen harshness and allow excessive bouncing, but they do not usually cause a persistent low ride height on their own. Loose wheel bearings affect wheel rotation and can cause noise or looseness, not reduced ride height or bump-stop contact.

This symptom points to an airflow routing problem rather than a cooling problem or a general airflow restriction. The component that directs air to the selected outlets (panel, floor, or defrost) is the mode door. If the mode door or its actuator fails, air may be diverted away from the chosen vents even though the blower is operating. A clogged cabin air filter typically reduces airflow out of all outlets, not mainly one outlet location. A temperature control/blend issue changes discharge temperature (hot vs. cold) but does not normally prevent air from coming out of a selected vent location. A failed A/C compressor would mainly cause poor or no cooling, not a vent-selection problem.