it could be the lifters. mabye...
cause when the lifters are at low rpm's, the oil pressure in them is low, causing (the bad ones) to make noise (dont know what you consider knocking)
anyways, when the engine is wound up (rpm's) the lifters harden because of the increased oil pressure in them and they quiet down. so try getting a stethiscope (sp?) or even a long ass screwdriver and holding it to your ear, with it against the motor to help pinpoint the problem.
just an idea, but it is woth checking out.
best of luck