Clacking sound in reverse and deceleration

Have a real odd noise coming out of what is otherwise a very quiet inline turbo 6. It started ONLY when in reverse. The engine makes a bit of a clacking sound (like I can hear the valves) and sort of sounds like a diesel engine. As soon as I shifted into drive or neutral, it went away.

My best guess is an exhaust leak that opens up when the engine orientation shifts a little while in reverse? It has begun happening whenever I take my foot off the accelerator too. Stops when I come to a stop, only makes the noise while coasting. However after the engine get to temperature, it usually goes away completely.

Would an exhaust leak go away as it heats up (things expand)? A heat shield rattle seems possible too, but that seems even less likely to go away when warm.

No CEL and nothing seems off in the OBDII readings. Possibly related, I just had a new cat and O2 sensor installed about 2 months ago. Any thoughts would be helpful. Oh, it is also louder inside the car. I can’t really hear any difference standing outside.

I think your exhaust leak idea is a good one, a likely cause. Shifting into reverse or accelerating, decelerating the engine causes the engine to rotate slightly in its moorings, which could indeed open & close a small exhaust leak. Since you have a new cat, those connections are a good place to start looking.

May also be the exhaust tapping something under the car due to close contact. There should be witness marks to help find it.

Just following up, it was the oil filled upper motor mount. Got lucky and found a video on YouTube of the same sound.

The telltale (compared to an exhaust leak) was that I could only hear it in the cabin, there was no noise at all in the engine bay or under the car, even when I put a hose to my ear and listened to the joints. The cabin itself was the resonator for the vibration. It also only appeared on deceleration and in reverse most of the time, but quickly got worse and worse. $150 later and about an hour’s worth of easy work, it is replaced and it sounds like a whole different car now.


Thanks for the follow-up OP. Since a mount is bolted to the body of the car, makes sense that a faulty mount would produce the noise in the cabin more than standing in front of the engine compartment, while an exhaust leak would do the reverse. The curve ball was the recent cat replacement which makes you think something to do w/all that, but that turned out to be a red herring. Good diagnosis!

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