[AbandonedBronco]

Hi All,
I'm having a reoccurring sound coming my drivetrain. I have no idea what it is, or where to start looking for it. Hoping you can help me pin-point it so I don't have to just start tearing things apart.

Symptoms:

• I'll be cruising along normally when an odd whine/vibration will very suddenly come from the transmission or transfer case. I can feel it in the shifter and the whole vehicle has a slight buzz to it. It's not a grinding at all, but kind of a strong but quiet buzz/whine. It's not severe and it doesn't impair driving.
• It's probably happened around 3 - 4 times in the last two months, so it's not a constant thing.
• It only happens at higher speeds. 65+ mph or so. I can drive it for weeks around town and it'll never occur.
• When the sound starts (or stops), there's no clunk, hard hit, slam, etc. The sound/vibration is just ... suddenly there. Pretty much just "...........................ghnngngnnngnnnngnnnngnn"
  Like someone flipped a switch.
• The only way to make it go away is to bring the vehicle to a complete stop. It seriously has to hit 0 mph and be fully stopped. Once the vehicle stops, the sound/vibration stops like it was never there and I can speed back up and head on my way.
• Putting the clutch in doesn't change anything.
• The intensity of the buzz/whine changes in relation to the vehicle's speed.
  

I have no clue what this might be, or how to go about pin pointing it.

The transmission is a 4spd/OD SROD and it was fully (professionally) rebuilt about 2 years ago.
The transfer case is an NP-208. It's solid and works great, but I don't know if it's ever been gone through.
I'm quite sure it's either the transmission or the transfer case.

Thanks for any help.

 

[A1Fiddler]

How old are your u-joints? They're the usual suspects.

 

[Jasonhughes2009]

that or bearings could be seizing somewhere, or not have the right/proper amount of lube, or the tranny could be high/low on fluid

at 65 when the noise comes on... does it come on when your accelerating to 65 or as soon as you hit 65 and let it coast?

 

[AbandonedBronco]

The U-Joints are pretty new, maybe just a few years. But I've had U-Joint problems before. This is nothing like that at all.

The best way to describe it is it feels like a ring or gear of some kind in the trans/transfer suddenly comes loose and is spinning freely around a shaft incredibly fast. Once the vehicle stops, it clicks back into place.

Having dealt with other drive-train issues before, it really does not feel like anything that has to do with drive-shafts or the like. It's something internal.
There isn't any grinding noises, sudden drag on the vehicle, etc. Just a whine/hum/buzz that I can feel, especially in the shifter.

 

[AbandonedBronco]

that or bearings could be seizing somewhere, or not have the right/proper amount of lube, or the tranny could be high/low on fluid

at 65 when the noise comes on... does it come on when your accelerating to 65 or as soon as you hit 65 and let it coast?

It happens when I'm coasting.
I'll check the fluid, but it doesn't leak and I checked it about a year or so ago. Worth looking into though.

 

[Jasonhughes2009]

read diff backlash probably needs to be adjusted then

 

[AbandonedBronco]

read diff backlash probably needs to be adjusted then

Is that something that needs to be done on the 9"?
How is that done? Is there any way for me to just look into seeing if that's it, or does a shop need to check that?

 

[Jasonhughes2009]

well... easiest way..... chalk the tires, and pull the 9 inch carrier and everything out, set it in a bench vise, paint a couple teeth on the ring with non drying paint, spin the driveshaft flange while applying resistance to the ring gear and take pics of where the paint wheres off of

go both ways with it, forward and reverse

good way to remember things, driveing is on the convex side of the ring gear, coasting is on the concave side of the ring gear or "caves on the coast"

 

[TheUnforgiven]

does the np208 have an oil pump in it, or is it just a splash oiling system?

Abandoned i don't think a rear end will make noise intermittently like that.

 

[Jasonhughes2009]

does the np208 have an oil pump in it, or is it just a splash oiling system?

Abandoned i don't think a rear end will make noise intermittently like that.

i only tell him this because mine had this problem to, granted 8.8 not 9, same concept though, could very well be something else

 

[TheUnforgiven]

i only tell him this because mine had this problem to, granted 8.8 not 9, same concept though, could very well be something else

i see your point. just weird to me that a rear end would only act up under such specific conditions you know?

 

[Jasonhughes2009]

i see your point. just weird to me that a rear end would only act up under such specific conditions you know?

Yeah. Mine only happened when it got really cold when we went to the snow. Damn near the whole way coming back was coasting down hill and it lasted for maybe 10-15 minutes. Once the fluid got really warm it was all good though.

 

[AbandonedBronco]

Thanks for the info on the differential.
Heh, simply due to the sheer amount of work it sounds like it'll take to check that one, I'll save that idea until last.

It also seems to be..., hrmm... "finer" than that. Like an alignment ring vs. big gears. If that makes sense.



As for the oil pump in an NP-208, I believe it does, but I don't know for sure.

 

[slw210]

Can you get the noise at a certain RPM in different gears? Is it only in Fourth gear? Does it stop at higher speeds/RPMs?

Wierd that it only stops when you come to a stop.

 

[AbandonedBronco]

Can you get the noise at a certain RPM in different gears? Is it only in Fourth gear? Does it stop at higher speeds/RPMs?

Wierd that it only stops when you come to a stop.

It's hard to say. I'm still usually in 3rd (don't use overdrive much with my 31" tires). It is also very inconsistent, so it just happens randomly. There doesn't really seem to be anything specific that brings it around. It just sort of happens when it happens.

I know that doesn't help much, but that's how it is.

 

[nickfrenier]

Hi everyone im new to this forum thing so i dont know if i did this right? I just got a 91 bronco and it tends to stall in reverse i have some hunches to go along with like egr valve replacement and IAC replacement but any other suggestions would be nice its got the 302 and E tranny i dont remember the name im not a ford guy so i know nothing about them

 

[SnowTom]

Hi everyone im new to this forum thing so i dont know if i did this right? I just got a 91 bronco and it tends to stall in reverse i have some hunches to go along with like egr valve replacement and IAC replacement but any other suggestions would be nice its got the 302 and E tranny i dont remember the name im not a ford guy so i know nothing about them

You need to start your own thread and not clog up someone else's when people are trying to help them with a totally different problem.

 

[SnowTom]

It's hard to say. I'm still usually in 3rd (don't use overdrive much with my 31" tires). It is also very inconsistent, so it just happens randomly. There doesn't really seem to be anything specific that brings it around. It just sort of happens when it happens.

I know that doesn't help much, but that's how it is.

How loud it the noise? Can you hear it over the radio or when people are talking? Can you give us some type of idea where it sounds like it's coming from? At this point it sounds like it could be a bearing anywhere or even a speedometer cable.

 

[mike85xlt]

yeah ive heard some GOD AWFUL sounds coming from a speedo cable before and "it only happens when it happens" kinda thing... when it does happen does it go away when you downshift? it kinda sounds like your not getting true full engagement of the gears at those higher speeds like the dogs on the gears are worn down or your shifter is sloppy... the shifters are junk on those transmissions...

 

[Mr Bell]

It sounds to me like the wear pads have worn off of the fork for your t-case reduction gear. The collar walking around ans intermittenly touching the reduction would do that.