Probably a leaking exhaust manifold,

There's still a certain amount of fuel in exhaust gas hence Cat's.

As the manifold is pre cat air leaking in creates a combustion with hot exhaust gas/fuel.

Probably nothing like I remember from way back,

A single cylinder BSA had this so bad it could knock a pint glass of a nearby oudoor bar table!!

Owner loved this so it went on for a good time.

Years ago working at a Saab dealer i remember being shown how Saab's of a certain era could be made to backfire extravagently, though it was pointed out to me that attempting to do this on a car without a stainless steel exhaust could prove unwise!. As far as i recall it, while driving along dip the clutch, give a few good hard presses on the throttle, switch the engine off then back on again, then, BOOM!. Not very sensible i know, but pretty funny to my juvenile mind. One time i was getting a lift home from a mechanic in his old 99 and he did it as we approached a young man walking along the side of the road, well the poor chap actually dived to the ground, probably thinking a bomb had gone off!.