'Popping' as the weather warms up

Looking for some guidance as to what to look at first.

I have dual Kadrons fitted and when decelerating the 'Sonny' pops/backfires a bit. This tends to be worse on hotter days.

I had the engine out a couple of weeks ago and am reasonably confident that the exhaust system is not leaking as I removed and refitted it all with new gaskets/sealant.

Can anyone help with a prioritised checklist to try and eliminate the excess noise? Could it simply be a timing issue?

Thanks in advance as always.

Assume it is a header.

If it has the stupid slip joints on 2 & 4 grab hold and push and pull. There should be no movement.