A few things to note about 68's, they are 3800hp, they provide a Electric Train Supply index of 96, which of each the coaches added up to much not exceed this number. When the ETS is switched off the RPM is much lower and thus lowers the noise, the issue with turning the ETS off is that most Mk5A systems rely on it. There is back up batteries availabile on the stock, however they are for crucial systems only, for example, lighting, heating. Toilets, HVAC etc do not function without it. The depot is fitted with a shore supply, however due to some illicit design detail (failure) it can only provide a supply to the coaches. It does not provide any 'pre-heat' to the engine. If the first train is due to depart the depot at 0510 then starting the loco at 0400 seems quiet sensible, (the noise maybe not) this giving the loco time to Pre Heat, lubricate and start cranking. The ETS then needs to be supplied to the stock before the train moves to check for any faults etc.
37's etc were much quieter. I have been stood at Man Vic next to a 66 on a liner and with a 68 next to it and the noise was considerably less on the 66.