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Sounds like you jumped the timing chain. BTDT.
I seriousely doubt this because this truck has been takin extremely good care of and there isn't anything that would suggest a piston faliure.Just a suggestion, is there any possibility that a piston has gone bad. As for the rattling once it's under load or put into gear, could that possibly be the front end of the tranny. Possibly one of the bolts on the bell housing was shaken loose or the torque converter bumped wrong.