I just went through this on my D1 and it ended up being the TIRE PRESSURE. I foolishly pumped the tires up to 44 PSI (labeled on the sidewall of the tires) and it would vibrate like crazy, mostly on cement highways at around 65-70 mph, then would go away if I went faster... Lowered the PSI down to 37 and it almost completely disappeared. Still one spot on the road where it vibrates so gonna drop air pressure a bit lower.