Having caught up with a two Highlander regulars yesterday, and my own limited experience of the new trains, I think that the two posts above have got it pretty much right. We have no technical knowledge by the way, we're just people who spend a lot of time travelling by rail and know a rough ride when we feel one.
Anything to do with couplings and connections, in other words horizontally generated movement and noise, has been improved out of recognition on the mk5s. So shunting, braking and acceleration has none of the old bangs and jiggles.
As for the knocking or rattling. Going by earlier posts on this and other threads, this is a problem with the way that the secondary air suspension fails to cope with track that's in anything other than perfect condition. The vertical motion created is causing an up and down knocking, but this is believed to be more common north of Edinburgh and large distances can be covered on the WCML at full speed with no disturbance at all. Fair enough, but a mk3 in good nick soaked up pretty much everything, so it isn't good enough.
I can't get anyone to be too upset about sloshing!