Chirping

[jmbrodsky@domain.elided]

     John Landry,
     
        Now you of allpeople should recognize that when you build pipes for 
     low Reynolds number flow like AIR, you get weird whistles and such due 
     to harmonics and reflections in the pipes themselves.
     
        Clues:
     
     1) You 'hear' it coming from the 'back' of the truck.
     2) 'The Noise' Seems only to emanate when the vehicle is in motion
     3) at about 35 mph
     4) you can't hear it from the engine bay
     
        Deductions:
     
     1) Prolly exhaust noise
     2) Benoulli's principles in action across the back (Outlet) of the     
       exhaust pipe
     3) Load dependant, 35 mph opposed to no load
     4) since it's exhaust noise
     
     
        My hypothesis:
     
        The end of the pipe is whistling while exhaust noise and 
     'puff-puff' is affecting the tone.  It happens at 35mph when the air 
     speed and the exhaust speed match the harmonics of the pipes.  And, 
     you are super critical of anything about this truck.
     
        When I got my 3" setup on the Travelall it sounded like a BOMBER!  
     Anyway I got used to it, and became quite aware of the engine drone 
     since now, it actually had tonic propoerties.  Next I put a K&N in, 
     and WOW, it makes the truck louder to the perception of the driver.  
     Super load on acceleration, louder on cruise, lower fuel economy with 
     K&N.  What should I do about jetting?
     
        If you can't hear the noise in the hood, try and block your wheels 
     and drive up against a curb or something, and put the e brake on, and 
     put it in gear, and make the revs and everything match the enviro of 
     the chirping.  Dollars to donuts it's the exhaust.
     
     BTW are the exhaust ports different between your truck 392 and your 
     scout 345?  oval/round/etc like a chevy?  Just curious, since the 392s 
     are usually rear dump, and 345 center, do you have an extra set of 
     rear dump kicking around now?  Please let me know.
     
        -Joel