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Q: Knocking/Rattling sound in the engine bay on startup.

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I was wondering if it could be a faulty belt tensioner, since the problem is intermittent. Note that the rattling/knocking sound goes away after the car warms up. So you'd only hear it whenever the car is starting up from a few hours of non-use/ an overnight's rest/ on a cold winter day.

What do you think? :)

My car has 130000 miles.
My car has an automatic transmission.

It would be difficult to tell what is causing the noise without first inspecting the vehicle using a mechanic’s stethoscope on various locations to determine exactly where the source of the sound is. However, it is possible that the noise could be coming from the engine itself or one of the engines belt driven accessories. I would recommend enlisting the assistance of a qualified professional to listen to different parts of your engine with a mechanic’s stethoscope to find the noise. Possible causes of this noise would be valvetrain parts within the engine, rod/main bearing out of place within the engine, pulley/tensioner fault, water pump, etc. Once the mechanic finds the noise in question, they should be able to provide you with a more accurate solution to your issue. If you would like, an expert from YourMechanic can provide the diagnosis of the rattling/knocking sound for you, at your home or office.

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