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[hitecdan]

Could be turbo drone. That's when the vibrations of the turbo hit the resonate frequency of the turbo pedestal or some part in the exhaust system. The resulting noise can sound like a hum or droning noise. Go a few rpm up or down and the noise stops.

This was an issue with eariler trucks and Ford had to redesign the turbo pedestal. Other causes were bound up exhaust components.

Here is the TSB on it:

6.0 Moan/Droan Between 1500-2000 RPM:
Engineering has investigated this condition and found the cause to be engine firing pulses causing the turbo mounts to vibrate/resonate. For trucks built through 9-29-03 install a revised turbo pedestal, P/N 3C3Z-6N639-BA. Trucks built after 9-29-03, install a turbo bracket A-brace, P/N 5C3Z-6N639-BA. If the draon is still present after installing the brace on trucks built from 9-30-03 through 1-17-05, replace the turbo pedestal with 5C3Z-6N639-AA. Ensure the droan is not caused by a bound exhaust system. Inspect for any damaged or broken exhaust hangers. Check the turbo downpipe by loosening the pipe at the top and bottom, checking its alignment and retightening. If the droan is still present, check the part number embossed on the turbo pedestal just below the turbo compressor outlet to the intercooler. If the number IS NOT 1846207C1, replace the pedestal with the above part. After replacement, or if the number IS 1846207C1, loosen the turbo inlet pipes and EGR cooler flange and neutralize by installing the components in this order: snug the flange between the LH and RH inlet pipes; torque the EGR cooler clamp to 53 in/lbs; torque the pedestal bolts to 23 ft/lbs; torque the turbo inlet clamp to 9 ft/lbs: snug the inlet pipes to manifolds. Ensure all components are properly aligned. Torque the flange between the LH and RH inlet pipes to 20 ft/lbs and the inlet pipes to the manifolds to 20 ft/lbs. Broadcast Message 0992, SSM 17530, TSB #04-9-4 & 05-19-12.

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