I managed to catch one out towards Wallyford earlier; they are very quiet at speed! Very smooth and no rattly panels, unlike some of the newer vehicles.
I'm also sure they sounded slightly different, even in Edinburgh between Princes Street and The Jewel; would regular running at speed do anything to affect how they sound? Or am I hearing things that aren't there?

Edit: Apologies, accidentally pressed "post" before I'd finished writing.
Don't mind me... its about to get geeky...
Regular running at speed, coupled to road conditions, specifically those bumps in Wallyford and some of the country roads in East Lothian appear to have taken their toll on some aspects of East Coast stock.
Id put money on a direct correlation between bad road conditions and speed running and rattles, engine condition etc.
Got to remember the structure of the vehicle takes on a decent amount of beating the faster it goes and environmental factors such as wind. Buses are, by their nature a lightweight design. So things do tend to rattle and even come loose.
They only heavy part of a bus really is the lowest parts of the lower deck, everything else is rather thin and flimsy, so that vs wind isn't ideal! Panels are really just thin aluminium and the odd bit of fibreglass.
If It weren't for its size, you could pick up the entire front end of a Gemini 2 (the outer skin that is) in one hand.
Ideally motorway miles would be done by a coach, something a little more rugged built.
But the types of routes these vehicles run and where they goto aren't necessarily suited to the length or weight of a coach, not to mention the reduced capacity.
Still, the Eclipses and Gemini's East Coast buses have are preferable to say an Enviro 200/Enviro400 in terms of rugged build etc, but not by much.