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Dull knock from rear with acceleration on highlander hybrid

My 2009 Toyota Highlander Hybrid makes an intermittent noise when I accelerate from a stop or decelerate to a stop. My best description of the sound is a sharp thud or a dull knock (maybe a dull pop). It happens almost exclusively when I accelerate from a full stop or from moving very slowly. A similar sound happens when I decelerate to a full or near stop. I’m very certain that the sound is coming from the rear suspension or drive train of the vehicle, but I have not proven it. When I’m moving along at normal speeds, I don’t hear any sounds and the car feels solid. By pure luck, going over the right sort of bumps causes a somewhat similar noise, but that might be a read herring.

Trips to two separate Toyota dealers have not fixed the issue since it is not easily repeatable. The first dealer tried tightening some bolts. The second replaced one of the front(!) struts, tightened bolts and lubricated things. I have managed to reproduce the issue for the second dealer, but the noise wasn’t persistent enough for them to track it down.

This type of SUV should be regarded as all-wheel-drive, but power is mostly delivered by the front tires. The rear tires are driven by an independent electric motor and differential. The front and rear drive trains operate separately; the only mechanical connection between them is the rest of the vehicle.

I’m wondering if the electric motor could be slipping a little bit.

Respectfully submitted to the Car Talk Community by Erik.

Taking the Hybrid system out of the questoin… It almost sounds like your rear breaks are sticking just a little bit… Then they “pop” open when you start to move… Has anyone checked them to make sure the slides are lubed well?

Maybe it is just the rear drive kicking in. It is a funny system since the front drive is the regular drive. Maybe for torque or other reasons the rear drive bumps as it starts. This would cause a noise just due to shifting the rear mounting points from passive wheels to drive wheels. The bump would be from the rubber frame mount points. This would be normal. But if it sounds like a metal bump then the bushings may have failed. These are tough to diagnose until you disassemble the mounts. Mostly this is an audible diagnosis while driving. It may be just one wheel doing it. The dealer may just be dismissing your verbal report for their own reasons. Also it could be a software error pushing the drive too hard and stressing the mounts. I would really beat on the dealer to find a fix. Call the zone rep if you have dealer issues. When bad computer controllers are involved the company knows about it but does not have a plan yet. You need to document the issue to be successful if this takes a year past warranty to fix.