Strange how Morris Minors make a trumpeting exhaust sound sometimes when changing gear, if being driven on high throttle. Not as common these days as the cars are older and being driven more gently, unless they have been fitted with a new or re-conditioned engine. Doesn't seem to happen with other cars. It would seem more logical for the exhaust to become noisier the faster it revs up, not when the engine is pulling back for a gear change
I think it's the piston in the S.U. dropping quickly that causes the parping in the exhaust. Tricking around with different grade oils in the dash pot improves it. We found 3in one oil was best.
I also remember that it used to happen at any speed when Minors were being driven by older people*in the 70's/80's (they had owned the Minor from new), because they didn't lift the accelerator when dipping the clutch to change gear.
*I'm trying to be nice, but TBH, it was always old dears who got their licence before there was a test, and who were never great drivers even in their youth.
It seemed to contradict how the power output seemed to shoot up when most cars were reducing revs at a gear change, but if the driver didn't ease off the accelerator when changing gear, the car would 'parp'. Nowadays the 'parping' happens very rarely. The Morris Minors still on the road are so old that the drivers are only revving them very gently
Stainless steel/reproduction exhaust systems have helped eliminate it too.