Thanks chaps.
Re class 20s and class 40s - They do sound very similar in some ways due to the fact that they’re both powered by English Electric SVT engines.
Later developments (37s, 50s etc) had CSVT engines (the C standing for Charge cooled - ie with an intercooler). That whistling sound is from the design of turbo charger they had fitted.
Follow the lineage and eventually you get to the turbocharger in the class 56 which is incredibly loud!
You could pretty much say that an 8 cylinder 1000hp class 20 engine is half of a 2000hp class 40 engine in some ways, with a lot of shared engine parts between the two.