Noisy X linear bearings (i3). Is there such a thing as a good one?

My X carriage is quiet until it has to move faster than about 50-60mm a second. You get a terrible racket from what sounds like the bearings (scratchy, nails on chalkboard with a metallic taste). I assume you get bad linear bearings, but do you also get good ones?

Also, do they benefit from grease? (almost said Greece, which would have made no sense whatsoever).

I believe they typically use oil, a noisy one could need lube, or it could just be bad.
However, there are plastic "bearings" such as Igus, however, Igus are expensive. Fear not, people have printed them (check Thingiverse), they really are much quieter. If you do print one, I would recommend ABS or Nylon, not PLA.

Recirculating ball bearings have a problem where two balls next each other rub because the surfaces are going in opposite directions. It gets worse at high speeds or loads. The common solutions are make every other ball smaller and only carry load on the large ones. THK actually uses a plastic chain like spacer to keep the balls from rubbing.

I've had a look at the Igus web site, they seem to know their stuff, but I'm pretty sure their kit is overkill. After you mentioned it I've had a look at printed bearings and I think I can do something pretty funky actually. I've ordered sacks of ball bearings, skate bearings and some other things of interest.

I'm figuring out OpenSCad at the moment, so hopefully I'll be able to knock up something decent shortly