Q: I have a 2010 Toyota Highlander and it’s making a rattling sound under the hood. It’s louder when idle. What could this be?

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Rattling noise under hood. It’s louder when idle.

My car has 100000 miles.
My car has an automatic transmission.

Louder at idle is a concern. At idle, oil pressure is at its lowest which particularly impacts the valve train. If the issue is not "within" the engine, other possibilities are the circuit of accessories that are driven by the serpentine belt. That "collection" is ruled in or out by disconnecting the serpentine belt and running the engine for a few SECONDS. If the noise disappears, obviously the source of the noise is sought from amongst that collection of components. If you request a vehicle noise diagnostic, the responding certified mechanic will pinpoint the origin of the noise and get this issue taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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