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"Rattle" coming from underneath when slowing down


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Feb 22, 2022
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Kansas City
I have a 2021 custom trail boss with a 6.2 L and the 10 speed transmission. Almost every time after I get up up to speed, when I slow down to about 15 miles an hour there is a rattle coming from underneath. I took it to the dealer, drove around with them they didn't know what it was. Then they replaced something to do with the torque converter(don't remember what specifically). When I got a back it was still making the same noise. About a week later my transmission completely went out. I just got it back after they replaced the transmission and it is still the same noise. After all that, the tech that worked on it said he had one and it is a normal noise. Has anyone had similar issue? If so did you get an answer?
I have a similar noise when I start up and yes going slow... I attribute it to the flappers at the muffler one more reason to go with a cat back system...
how many miles ?
4700, I actually just got it back today after a full transmission replacement. Unrelated, as they tried something with the torque converter first. That didn't fix the noise, then a week after that there was a catastrophic failure in the transmission. But the noise still persists. I honestly think what they did with the first repair caused my transmission issue.
I've not experienced that when slowing down but do hear a slight rattel on cold starts! Been suggested that it may be the exhaust near the dual outlets. Sounds more underneath to me though?
My 2021 5.3 10 speed has this rattle when coasting and the engine RPM's are around 1500. The vehicle has a cat back exhaust, so the adaptive valve is gone. I have been under the vehicle and did not find any loose heat shields, brackets, clamps, or anything else. A bit concerned that it may be in the transmission. The truck was driven all day, so the cold start rattle also does not apply.
I had a similar rattle when at idle or just around 1500 rpm. Had to have someone hold the pedal to replicate the rattle while i got under to look. Eventually pinpointed the rattle to BOTH Downstream O2 sensors coming loose and just rattling around in their spots. Very strange how they managed to come loose.

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