I have a similar noise, but it only appears when my motor is low-idling, or if I drive over a gradual hump (like an intersection) or turn sharply to the right. At first I thought it might be the after-market exhaust, which is tight in there. But I'm leaning towards rod/bearing issues.
It even can be heard when in park, if the car's at the right upward angle. Very irritating. But it always disappears when the rpm's go above 1k.