Thump from exhaust like a pre-amp

[AZ_Adventure_Clicks]

This sound was happening before my new motor was installed and it's still doing it- a strange thump that sounds almost like a preamp turning on after Auto start stop and on initial startup. It comes from the rear of the truck.
I replaced almost everything... Something with the cat maybe?

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[yoda13]

Mine does that too, sometimes. It’s not consistent, but unlike my other noises, I have not been able to narrow down a consistent set of parameters as to when it does it.

 

[AZ_Adventure_Clicks]

The most plausible is unburnt fuel in the exhaust, but I'm having trouble understanding how that can be... Especially with a brand new motor.

 

[yoda13]

That sounds quite ominous. I was hoping for something different, explanation wise. Mine only does it sometimes. You would think this would be a more constant thing for me, were that to be the case.

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