i had the same sound in our 2.7, it was slightly low on oil (less than half a quart), so i topped it off and revved it a lot. trying to go from low oil pressure to high quickly. i came to the conclusion that something, most likely sludge broke loose and plugged an oil supply port to the head. you might try some seafoam in the oil and change it again within 500miles.
btw, it went away after about 30 minutes of hard driving and revving.
it was really my only choice besides tearing the engine apart, wasn't an option at the time.