Some '98-'00 4.0L OHVs are prone to giving off a "marbling" sound at idle.
In '98 the pistons were redesigned in order to reduce the engine's overall recipricating mass and hence improve NVH. Unfortunately, this change resulted in the piston being able to move around in the bore which can sound like marbles rattling around in a can. Here's what Ford has to say about it:
15226 - 4.0 OHV MARBLE NOISE
Some 1998-2000 Ranger/Explorer equipped with a 4.0L OHV engine may exhibit an engine noise that may be perceived by the customer as a "Diesel" type noise and is known as Marble Noise. Marble Noise is clearly heard when the vehicle is at idle, under 2000 RPM or under very light/no load. High mileage studies of the condition show no adverse effect to engine life or performance. Cleaning carbon from the combustion chamber will have no effect on the Marble Noise condition. Marble Noise is a characteristic noise of this engine and cannot be serviced with an engine exchange.