Hi guys.  I got a weird one this time.  My 2000 Legacy wagon M/T 230k has developed a knocking / thumping / vibration noise that only occurs under certain conditions.  First the engine must be warmed up a lot - not just operating temperature but 5+ minutes of freeway driving.  The sound will begin as a tiny vibration like a tire lost a weight or started to go flat.  Then the vibration will get stronger and the knocking in the engine background noise becomes more distinct.  The vibration can be f