Rattling noise

Went on a hunting trip today and half way through the morning notice a rattling noise comming from what sounds like the front diff on my 450.only happens around 10 to 15 kmh.checked it all over including the bearings all seems good and axles.sounds like a pile of rocks in the diff bouncing around.change oil and never say any metal and it was clean.and body ever have this happen or know what it could be.?

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