Symptoms
- Rattle or vibration noise present in Drive or Reverse at idle
- Noise disappears or reduces significantly in Park or Neutral
- May be accompanied by a slight shudder felt through the car
- Could be more noticeable with the AC on, adding load
Likely causes
- CommonWorn motor mountA worn mount allows the engine to shift slightly under the torque load of being in gear, causing contact with nearby components.
- CommonWorn transmission mountSimilar to a motor mount, this can allow excess movement specifically under drivetrain load.
- PossibleLoose heat shield or exhaust component that only contacts something under slight engine movementThe small amount of engine shift when in gear can be just enough to cause intermittent contact.
- PossibleWorn transmission mount bushingA specific bushing component wearing out can cause this exact symptom pattern.
- Less commonLoose accessory bracket that only vibrates under loadLess common, but possible if other mounts and shields check out fine.
What to do
- Have someone watch under the hood (from a safe distance) while you shift between Park, Drive, and Reverse at idle to see if you can spot excess engine movement.
- Note whether turning on the AC (which adds load) makes the rattle more noticeable, which supports a mount-related cause.
- Check motor and transmission mounts visually for cracking, separation, or fluid contamination if accessible.
- Have a mechanic perform a 'mount check' by having someone hold the brake firmly while briefly applying gas in Drive and Reverse, watching for excessive engine movement.
- Check for exhaust components that might only make contact when the engine shifts slightly under load.
- Have worn mounts replaced, since this is a fairly common and moderately priced repair.
- Avoid ignoring worn mounts for too long, since they can also contribute to other symptoms like clunking when accelerating or shifting.
- Get all mounts checked, not just one, since if one has failed, others are often not far behind depending on age.
Tools and parts
- Flashlight
Motor mountSearch: “motor mount for [year make model]”
Transmission mountSearch: “transmission mount for [year make model]”
When to call a pro
A rattle specific to being in gear is efficiently diagnosed by a mechanic doing a mount check on a lift; this pinpoints which mount, if any, needs replacement.
Safety
Use caution
Keep hands and body clear of the engine bay while someone else is applying gas during a mount check.