2004 Exhaust Moan

[speede]

After warmup I'm getting a fairly loud moaning sound from the exhaust system. There seems to be a TSB on the issue, but I can't seem to find a cause for the noise or a solution. The moaning is noticable under light throttle. Any ideas as to the cause?

 

[ELVATO]

What do you mean by moan? Is it constant (as is it's always doing it?) Also, does it increase with RPM, or stay at a constant tone.

 

[speede]

The "moan" is the term that was used in the TSB, but I'd describe it as a hoot or low throaty rumble. The sound disappears at idle and at high rpms, and the engine seems to sound and perform fine. The sound doesn't seem to increase in volume or change tone at higher/lower rpms. It sounds like a hollow sounding resonance within the muffler. I just hate to replace the muffler if the resonance is being caused by the cat./EGR/Engine etc.

 

[mikegigabyte]

Have you modified the air intake, on an old car years ago I put in a different intake and it made the engine sound louder because the stock uses a muffler in the tubing to keep it quiet.

Also on my expedition I had a hose running over the plastic engine cover rot away and the vacuum made the same type of noise at idle.

 

[speede]

No modifications, and I've checked for vacuum leaks. The only vacuum related item seems to be that I can hear vacuum bleed off rather quickly when the engine is turned off which seems to be associated with the IAC valve adjustment of engine vacuum. The sound is definitely associated with engine load and rpms, and the sound and vibration are magnified in the exhaust system. The sound is not apparent at all in the engine compartment, and the intake/ehaust manifolds seem fine, and I don't have any exhaust leaks. Here is the TSB that seems to match my symptoms.

Component Description:
83. ENGINE AND ENGINE COOLING:EXHAUST SYSTEM Bulletin Number: 17135
Bulletin Date: Sep 2003

Vehicle: 2,004 Ford Expedition
Summary:
MOANING NOISE FROM THE EXHAUST SYSTEM, OR VIBRATION IN THE REAR PASSENGER FLOOR PAN, WHEN DRIVING 45-55 MPH AND 1200-1400 RPM ALL WITHOUT FLEX COUPLING IN THE EXHAUST. ( NHTSA ITEM NUMBER - 10004051 )

 

[speede]

I'm starting to think that the moan is drivetrain related and may be bad driveshaft u-joints, spline or carrier bearing. Will any of these make a moaning sound. The sound seems to be present at 2000 rpms at 65 to50 mph, usually uphill rather than downhill.