Here is what I think. YOu have been saying that the noise has been there ver since you bought it. It sound like a Timing chain/belt, not sure what SVTs have, in genereal, car with higher milage get it, over a period of time the chein streches and there is more playtime, therefore it might be hitting something. IT goes oway when you rev up the motor. JMO but don't just rely on me. Take it to a mechanic. Rodnock would not go away after the car warms up, and I think it would make noise in the upper portion of the engine.
I had a 94 altima and it had the same thing.
