I think the noisiest and best sounding engines were the 20,s, 33,s, 37,s, 50,s 55,s, 56,s 58,s, the older engines are the best for pure noise and hellfire thrash. I have some memorable thrash from various engines, had a 31 and 26 mega thrash on a 2 day railtour, got looped at the bottom of shap and the noise they made up Shap was unbelievable, sounded like they were ripping themselves apart, had a pair of 20,s diverted on a railtour down the GSW doing 85mph clocked, was absolutely hellfire, had pair of 33,s down the WCML from Glasgow to Birmingham on railtour which was great thrash all the way.
Been fortunate to have some great thrash and high speed blasts from class 55 Royal Scots Grey, on the ECMl and WCML, head out window enjoying the thrash, definitely a hellfire beast, the 56,s or 58,s could make a good racket too, and really thrash, had some really good thrash and high speed running from 50,s on railtours and service trains, loved the Hoovers, but my most travelled behind and probably favourite were the 37,s which were just pure thrash monsters, too many hellfire thrashes too remember them all, when double headed they could get very high speeds too, had over 100mph out of them, a memorable run was Fort William to Exeter on a land cruise few years ago, at front of the train, running very late, ended up being over night journey, hours upon hours of blissful thrash.
For modern trains and locos, the 68,s aren’t too shabby, and the 66,s are ok but can take or leave them, I think the Stadler Flirts 755 make a decent amount of various noises when switching electric to diesel and their engines are pretty loud, but nothing can compare to older generation diesel locos, just make better noise and sound.
