I've just read your post
I hadn't been thinking about the sound you are describing, but reading your post has brought it vividly back to me, together with the memory of the clouds of exhaust
As Roger would say - dreadful!!
Mind you, I suspect a lot of this "romance of the dmu" is a bit rose tinted, I also remember thinking they were clapped out rattletraps, but of course that was before I had had experienced the pleasures of the "Pacers"
Back to the nostalgia - I also loved the way that the driver would be desperately trying to find the next gear as you heard speed dropping by the slowing "tee tum, tee tum" of the wheels on the rail joints, followed by the gentle "kick" of engagement and protesting from the engine as it picked up revs from somewhere near stall to "full exhaust raspberry" revolutions and contemplation of further gear changes.
The whooshing of the brake applications..... the swinging of the drivers bag hanging on the partition....
Bee Barp!
Simon, back on the train to Swindon/Severn Beach/Bridport/St Ives/Weymouth/Wenbridge Junction.....