Stupid question, maybe, but are you sure it's the engine knocking? I get what sounds like a valve clatter under load also, but aren't our cars supposed to have all kinds of fancy knock sensors and such to prevent knocking? I think my noise might actually be one of the heat sheilds under the VX rattling around when the engine is under load and vibrating a bit. I haven't gone down the road with my head hanging out of the window to try and pinpoint the noise yet, so I could just be talking smack.