Droning noise when letting off gas at highway speeds

[Anthony]

I was on a longer drive this past weekend on the highway and (for once) had the ratio off. I noticed that in the 70 mph + range when I let off the gas, I heard a droning sound from what sounded like behind me. The tone / sound almost sounded like when you drive over rub strips (but certainly not that loud). “mmmmmmmmm”

As soon as I put my foot back on the accelerator, the sound would go away. It would only make the noise for a couple seconds (perhaps once it gets below a certain speed?)

At first I thought it was my new tonneau cover making a weird noise in the wind, but that doesn’t make any sense (only when letting off the gas?).

It isn’t loud and you certainly wouldn’t hear it unless it was perfectly quiet in the cabin, but I’m kinda anal when it comes to noises I can’t pinpoint the source of.

Differential noise? Driveshaft vibrations? Aliens?

Thanks folks!

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[Mokume]

I was on a longer drive this past weekend on the highway and (for once) had the ratio off. I noticed that in the 70 mph + range when I let off the gas, I heard a droning sound from what sounded like behind me. The tone / sound almost sounded like when you drive over rub strips (but certainly not that loud). “mmmmmmmmm”

As soon as I put my foot back on the accelerator, the sound would go away. It would only make the noise for a couple seconds (perhaps once it gets below a certain speed?)

At first I thought it was my new tonneau cover making a weird noise in the wind, but that doesn’t make any sense (only when letting off the gas?).

It isn’t loud and you certainly wouldn’t hear it unless it was perfectly quiet in the cabin, but I’m kinda anal when it comes to noises I can’t pinpoint the source of.

Differential noise? Driveshaft vibrations? Aliens?

Thanks folks!

Does the noise change in volume or pitch in relation to vehicle speed?
I see that you have a S/Crew and that you hear the sound behind you, can you duplicate the noise with someone sitting in the rear seat listening?
You don't hear it at all in the dashboard area?

 

[Anthony]

Does the noise change in volume or pitch in relation to vehicle speed?
I see that you have a S/Crew and that you hear the sound behind you, can you duplicate the noise with someone sitting in the rear seat listening?
You don't hear it at all in the dashboard area?

Thanks for the response. Let me get it out tomorrow and see if I can record it. Don’t seem to be from the dashboard area.

 

[Mokume]

Thanks for the response. Let me get it out tomorrow and see if I can record it. Don’t seem to be from the dashboard area.

I do remember the original Fox body cars back in the late seventies-early eighties making a similar noise, but it came from the dash area.
The problem was coolant flowing through the heater core at such a rate, that when you coasted (throttle closed) the change in liquid flow dynamics would create a slight droning noise emanating from the core.
The HVAC systems in those cars was designed so that coolant always flowed through the core, temperature modulation was controlled by a blend door instead of the more commonplace (back then) heater valve.
Let me see if I can dig out my old and tattered Ford NVH diagnosis manual.

 

[Anthony]

I do remember the original Fox body cars back in the late seventies-early eighties making a similar noise, but it came from the dash area.
The problem was coolant flowing through the heater core at such a rate, that when you coasted (throttle closed) the change in liquid flow dynamics would create a slight droning noise emanating from the core.
The HVAC systems in those cars was designed so that coolant always flowed through the core, temperature modulation was controlled by a blend door instead of the more commonplace (back then) heater valve.
Let me see if I can dig out my old and tattered Ford NVH diagnosis manual.

As a prior owner of 5 fox-bodies I applaud your memory!

 

[Mokume]

As a prior owner of 5 fox-bodies I applaud your memory!

LOL, I worked at a Lincoln-Mercury Dealership from 1976 to 1989 and became pretty good at locating squeaks and rattles.
The Fox bodies weren't half as bad as the Monarchs (Granada) when it came to rattles and squeaks within the interior.
Those cars provided a symphony of noises (in A flat, of course, and surround sound, I might add), all courtesy of FoMoCo...

 

[John Lyman]

I have noticed a similar noise a couple times - the first time was the backseat was not secure completely and it made a sound similar. Also, this week I had another weird noise kept thinking it was the kids car booster. I moved it and still the sound. Finally figured out it was the seat belt causing the rattling. It was pointed outward versus lying flat it was the most annoying sound that drove me mad until I found it.

 

[Anthony]

I have noticed a similar noise a couple times - the first time was the backseat was not secure completely and it made a sound similar. Also, this week I had another weird noise kept thinking it was the kids car booster. I moved it and still the sound. Finally figured out it was the seat belt causing the rattling. It was pointed outward versus lying flat it was the most annoying sound that drove me mad until I found it.

Now that noise I’m used to. I know whenever anyone has say in the front seat to fix the seatbelt first thing afterward. This is almost definitely not an interior sound.

 

[Toytec]

Be sure the lane departure warning is off when doing your testing.

 

[Anthony]

Be sure the lane departure warning is off when doing your testing.

Thanks for the reminder, but trust me....that is NEVER on.

 

[Mokume]

gear backlash kinda noise?

Certainly plausible, my well worn Ford NVH manual does list this as a possibility, however, the noise is described as a "moan"... I thought Halloween was over with?

 

[Mokume]

Be sure the lane departure warning is off when doing your testing.

Pardon my ignorance sir, but how does this effect or affect the issue that the Gentlemen is experiencing?

 

[VAMike]

Pardon my ignorance sir, but how does this effect or affect the issue that the Gentlemen is experiencing?

there's a setting to produce a low "rumble strip" feeling in the steering wheel if you drift out of lane

 

[Anthony]

there's a setting to produce a low "rumble strip" feeling in the steering wheel if you drift out of lane

Yup, that is correct.

I do want to add, I feel no vibrations, it is totally just a sound. Going to try and record it today, but my phone may not pick it up....like I said it is not loud, but you can definitely hear it.

 

[P. A. Schilke]

Yup, that is correct.

I do want to add, I feel no vibrations, it is totally just a sound. Going to try and record it today, but my phone may not pick it up....like I said it is not loud, but you can definitely hear it.

Hi Anthony,

Two thoughts....possible the Blowoff Valve? An Exhaust resonance maybe

Best,
Phil Schilke
Ranger Vehicle Engineering
Ford Motor Co.. Retired

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