Deceleration rumble

[JOHN CT]

For years I've been fighting with a rumble when shifting in between gears and coasting down hill.Its not backfiring just annoying fant popping it makes the bride nervous. I've timed and retimed, changed carburetors ( all different types) replaced the manifold and intake gaskets, and lm using electronic ignition. Checked for vacuum leaks with the wd trick no change in idle. What should I look in next ?