Exhaust leak after 3.0 replacement motor, but disappears when truck is in regen (Under 4k)

Had the dreaded thrust bearing issue with my new 3.0 duramax. I was without the car for almost 7 weeks while the dealer was completing the replacement . Since i’ve had the truck back it’s had an exhaust leak issue. ( no
cel) I’ve been waiting to bring it in due to no available loaner vehicles at the dealership I purchased it from.

What i’ve now noticed during this time is when the motor is in regen the leak seems to go away, but once regen is complete it comes back again. It sounds like a blow off valve every time I let off the gas. So now i’m wondering if it is an exhaust leak, or something else that was incorrectly installed during the replacement.

Any insight is appreciated . I tentatively have a service appointment scheduled for this upcoming week.