I had a close look at my 6 speed whilst it was up on stands for a spanner check last week. It is adjustable laterally, a little and I remember taking quite a bit of time getting it exactly central. It's well clear of any rails; there is a bulge, forward on the left side of the box that is very close to the insulation, possibly touching, but I can't say I've noticed any undue vibration. Is your box clear of the insulation? Is the vibration still high frequency?