Hi Steve & welcome to the forum.
Love your avatar. I never got to drive Thumpers but I always loved the sound of them. Whenever I was at Selhurst prepping an EMU, you could often hear them ticking over in the distance without being able to see them because of the density of units which were stabled.
And when they pulled away it sounded like an old fishing trawler was chugging slowly away from the depot towards Norwood Junction.
Best wishes, BW