Harmonic rumble at specific speed


Old 04-10-13, 02:56 PM
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Harmonic rumble at specific speed

Anyone have experience with a harmonic, oscillating (mild) vibration or rumble in a vehicle...? It doesn't increase or decrease with speed or RPMs. Occurs between 50 and 60 mph, most audible at 55 mph. Not tires....inaudible coasting in neutral. Must be in the engine.....balancer...?
It's a 4 door Ford Ranger, low miles.......runs well.
Thanx for any input....
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Old 04-10-13, 03:10 PM
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Nope, haven't had that one. Can you tell whether it's in front of you, behind, one side or the other, ...?
Old 04-10-13, 09:15 PM
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my guess would be a week motor mount. and 55 is the perfect torque/load ratio to let the vibration through.
Old 04-11-13, 04:02 AM
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Charlie: it's out in left field, but do you carry a ladder on the truck? Oddly, if a ladder is set almost perpendicular to direction of travel all the hollow rungs will act like flutes and will set up harmonics as your speed increases.
Old 04-11-13, 04:53 AM
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That takes out a lot of suggestions myself and others would have...
Motor mounts are possible, but so is the transmission and many other things.
Had it not been for your nutral test, I would have said tire balance/air pressure. 55mph is a sweet spot on many vehicles for vibration when one tire is not balanced with the other.
What year is the truck?
Old 04-11-13, 06:39 AM
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I get something similar when riding with my sunroof open on my Suburban. It is the sound of the wind mixing with muffler rumble (I have somewhat louder pipes). Only happens during acceleration. I also hear it if I should pull up to a traffic light next to another truck with similar pipes. Like a two part harmony....
Old 04-11-13, 08:32 AM
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Thanx for all the suggestions folks. It's a 2002 Ranger; I carry nothing in the bed; Hadn't considered a motor mount....will check them...if the damn winter ever decides to quit here in WI. (We're 15 to 20 degrees below normal); There are times when it's more audible than other times....on the same road. It seems to be more of a sound phenomenon rather than a physical vibration, which is why I termed it a harmonic oscillation; Didn't know what else to call it.
Again, thanx for the input.
Old 04-13-13, 07:12 AM
Join Date: Jan 2007
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Had a similar situation on a chev pickup. Jacked it up and ran it to speed that it would manifest its symptom, checked the engine RPM them ran the engine up to that speed and it was there. Turned out it was the exhaust system in a bind.
Old 04-13-13, 07:30 AM
Join Date: Mar 2013
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A friend of mine had a problem like that with a Ford Mustang and had to get a cross pipe installed he had flow-masters. Some how the cross equilizes the exhoust pressure if you have true dual exhaust

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