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Mystery Noise

188 Views 3 Replies 2 Participants Last post by  trekman
Hoping that someone might have some insights into a mystery noise on my 99 F350 7.3 4x4 with 275,000 miles
The noise is short duration and intermittent. So far it has only happened while driving. It may happen several times within a short period and then not at all for a while. It has happened while going downhill, flat and uphill. Seems to occur at steady throttle. The sound does not change with more or less throttle, although it is hard to tell since the noise is such short duration.
The noise comes on suddenly and sounds like compressed air.
The noise is like when you drive next to a wall with the window open and it turns on and off just as suddenly as when you pass a short wall.
I have a US Gear Decellerator exhaust brake and the noise it kind of like the exhaust brake, although I turned on the exhaust brake while the noise was happening and the exhaust brake made it's normal sound. Could it be a problem with the blow off valve on the US Gear? I checked the linkage on the US Gear and it had no play. Is there any other way to test it? US Gear seems to have gone out of business. They are not at their old phone number.
I have checked all my fluids including the automatic tranny the front and rear diffs which are full of oil and look clean.
I checked both my idller pulleys and they had no play and were well greased.
There is no play at the cooling fan and no leak on the pump housing.
The noise happens with AC off and the radio off.
I have a ADS 6 Position engine tranny programmer which I normally run in the lowest 5HP performance position. The noise happened in that position as well as the stock position.
The truck has a trans cooler and large trans pan and the tranny was rebulilt about 110k miles ago with performance parts. Other then that the truck is stock.
I made a recording but the it is hard to hear the noise. In the recording it sounds very much like a oncoming car passing with the windows open.
Thanks for any thoughts you may have.
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Interesting, do you experience a loss of power when this happens? Like a loss of boost pressure? If it's not a boost leak, it's likely something wearing out with that engine brake system.
Interesting, do you experience a loss of power when this happens? Like a loss of boost pressure? If it's not a boost leak, it's likely something wearing out with that engine brake system.
No loss of power or any change in performance. Yesterday it happened more frequently. Yesteday I was able to coast down a hill in neutral at idle and the noise kept happening although still intermittant shortduration. Revving the engine in nuetrall did not change the noise. At idle with less engine sound wgen the mystery noise happened could hear an underlying noise like dragging something on the ground which was intermittant.
I could find nothing loose or hanging down under the truck
Here is an update on the mystery noise. It is now more frequent and longer duration but still intermittent. It happened while coasting down a hill in neutral with the engine on. Revving the engine did not change the noise in any way. Makes me think it must be in the drivetrain since it happens in neutral.
Still no loss of power and don't feel any hesitation or glitch when the noise happens. Forescan says no trouble codes.
Noise is most likely to happen when going downhill with exhaust brake off or on.
I drained the transfer case and the fluid looks good which was changed about 10k miles ago.
Drained the tranny and fluid also looks good with slight mush on magnet also changed about 10k miles ago.
I did not drain the rear gear yet.
No play in universal joints or carrier bearing which were also replaced about 10k miles ago.
No play drive shaft at rear gear.
Jacked up rear end and wheels spun freely in neutral.
Ran truck in gear with wheels up while changing gears but no noise. Also tried braking while accellerating, still no noise.
I made this recording of the noise while sitting in the bed of the track with a friend driving it downhill in first gear.
What is it????
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