I had that same noise.. at about 2500 rpm.. tried changed oil and tried different oils... on the spark plugs i only use autolites in all my fords, but back to the noise.. i could not get mine to stop making that noise so i pulled the motor.. upon teardown found, pistion slap on all 6 and crank was hurt some.. machine work and new pistions and put back together and is now runing like a champ

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Mikey..
91 4x4 ext cab; 3" body lift: 4.0 OHV, 32X12.50 bfg's
will soon be named "bumblebee"