Mountain Bike Rider: Chain to long? Tension not right?

My son's Dawes (my son is 10 and rides a Dawes Bullet with 21 spd Shimano gearing incl EZ fire shifters btw) has always made a lot of chain rattle noise, etc as I have commented on before. His gears all change fine though. Pretty slick across the back and the front tbh. A LBS checked them over a couple of months ago too.

However, yesterday I took him on some different trails and one of them involved his first real tough short sharp climb and he went to use the granny ring for the first time. However, the combo he used, granny ring to middle of the block (7 spd) and the chain was practically dragging on the chainstay! Only the tension of his pedal stroke kept it from sitting on it. He changed to a lower gear and the bigger sprocket combo helped slightly.

Could it be the chain is to long? Not sure what else it can be as the gears all work smoothly. For eg, when in the middle ring at the front the rear block can be used smoothly and easily.

Any ideas?

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I have a singletrack mind.............

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