My 5spd xB has developed an annoying habit of grinding a good bit when shifting into 4th quickly, especially from 3rd. If I give it a momentary pause it's fine or even just rev it a little with the clutch in between gears and it's fine. Seems like the syncro's aren't matching up rev's quickly enough. Is this cause for serious concern? When I bought the car I got a 2yr/24k extended warranty so this will be covered I would imagine.