I have a 1999 F150 with a 4.6 engine. I had the DPFE switch replaced and new spark plugs replaced to correct a misfire and a low flow to egr code fixed. Truck ran fine for 3 days and then i started getting a misfire on cylinder 3 and 4. After a day or so of that i noticed a loud rattling rhythmic sound coming from the same side as cylinder 3 and 4. Its gotten louder and the misfires are getting worse as the motor warms up. Ive had it checked by 3 mechanics and all say it doesnt sound like engine noise or valve train noise. 2 of them say its loudest from the lower intake when a stethoscope or similar tool is placed against the motor and listened too. Is it possible its something in the lower plastic stock intake thats broke loose and vibrates in sync with the rpm's and cause the misfires as well? Also before the plug wires were changed I had misfires in those 2 cylinders as well. "Never heard that sound from a 4.6 before" and "We can take off the lower intake and see if thats it" is really all anyone says. Not ready to spend 3.5 4 hours of labor costs on exploratory surgery.