What could be a rattle sound coming from the right side of the engine?

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I have had the car for 5 months and had the valve gaskets replaced and the car tuned up. Even before the repairs, i have heard a small rattle on the right side of the engine, sounds like close to the glove compartment. I dont hear it when im outside of the vehicle, only hear it when im inside. The sound is not always there: it goes away when the car has been completely warmed up and driven for 30+ minutes. When i do here it is driving around town and the rattle gets more rapid and faster as you speed up, and if i take my foot off accelerator the rattle slows down. It sounds as if there are small pebbles being rattled in a small area in the engine. When engine is at idle its costant and steady rattle...when in Park it goes away. Any idea what this could be? I have been reading a lot about heat sheild rattling...but not sure if its that.