engine noise

Few days ago my 95 Caravan with the 3.3L and 100000miles started to make a knocking noise. I can only hear this noise when the engine is warm and at idle speed with the transmission in gear. When I put the tranny in neutral or park the noise goes away. I can not locate the source of the noise, but it’s coming from the engine. If I increase the engine rpm there is no noise. I suspect the timing chain or sprocket, but I’m not sure. Any idea or suggestion would be greatly appreciated. Thanks.

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