Howlin’ Wagon

[D.E]

Looking for some more armchair mechanics!

My M47 240 has had a high-pitched howl that increases with speed for the last five years or so. It’s hard to hear where the sound is coming from since the whole wagon interior acts like a resonance box.

It’s usually when it’s cold outside, and only for the first few miles and then it goes away. It changes with speed but is present in all gears including neutral.

I always just assumed it was the diff bearings until I swapped the rear axle for a LSD unit with fresh oil this summer and the noise persisted. Then I changed my focus toward the driveshaft center bearing. Well, that is now replaced as well with no change. I replaced the transmission fluid with fresh ATF at the same time.

So what do we think? Could it be the gearbox itself? Did I swap in another bad rear axle? It didn’t howl in the donor car as far as I can remember.

 

[the_hairy_baboon]

Don't know enough to point to part XX and say "that", but here's some isolation tests you could do.

1. Get up to a speed where it's oboxious. Change gear, maintain vehicle speed. Does the sound change or remain constant? (constant means not engine as engine speed has changed, change means gearbox/engine)
2. Get up to speed and pop into neutral, coast down, letting engine idle. If sound changes with speed, it's gearbox/back (but you're sort of out of options for back, beyond gearbox.
3. If you have an overdrive, activate it, get back to original engine speed - noise worse or better or same? Noise different means it's somewhere in the back half of the transmission/overdrive. Noise same means it's the main part of the transmission.
4. Start your car, idle in Neutral until you're up to temperature, then go for a drive. Noise better or same as your typical experience? Better - engine is the source, same - transmission.

 

[D.E]

Yeah, I’ve been doing those kinds of tests over the last 10000 miles or so, except for number 4.

As I said, it does not change at all with engine speed or gear. Only with vehicle speed, getting louder the faster I go. But it’s kind of intermittent and sort of comes and goes as I drive along.

I can be in any gear, or even neutral, with the engine at 800 or 5000rpm and it will make the same noise as long as I’m going the same speed.

I guess the real question I have here is; can the gearbox actually be causing the noise?

 

[D.E]

Forgot to mention that pushing the clutch pedal down makes no difference to the noise whatsover.

 

[Souped]

Are you sure it's the gear box? I'm wondering if it is something catching the air. The ladders on my truck whistle while I drive.

 

[eng_light]

It might be the output shaft bearings in the transmission, do you know if the transmission was ever run low on oil or was very old oil used?

 

[D.E]

Are you sure it's the gear box? I'm wondering if it is something catching the air. The ladders on my truck whistle while I drive.

Hey, good point! I’ll try it without the roofrack on. It didn’t have one when it started howling but it’s been on since I swapped the axle.

It might be the output shaft bearings in the transmission, do you know if the transmission was ever run low on oil or was very old oil used?

I wouldn’t be surprised if the oil I drained was original, 35 y/o 150k mile oil. There was something in there at least but it was pretty black.

 

[GTJordan]

When you changed out the diff, did you change the bearings on the axles? Those things can absolutely HOWL when they go bad.

When driving a steady speed, if you rock the car side to side with the wheel, does the pitch change?

Jordan

 

[the_hairy_baboon]

When you changed out the diff, did you change the bearings on the axles? Those things can absolutely HOWL when they go bad.

When driving a steady speed, if you rock the car side to side with the wheel, does the pitch change?

Jordan

If you take a turn at speed does the sound change? Left and right wheels will go at different rates to some extent. But I guess i'd question whether a bearing would stop howling once warmed up.

Trans fluid - worth checking the fill level to make sure it's properly filled

Now that we know it's intermittent when it does it (but only initially through a drive), then once warm it stops doing it entirely, and is truly only f(speed) that helps narrow down where it might be.

A crazy number 5 would be to mark your transmission output shaft bolts, mark the corresponding driveshaft bolts, separate them (and give enough clearance so they don't hit. Put the car in gear, spin the trans - get your output shaft up to the speeds where it normally presents, see if it still makes noise (lateral/longitudinal loading of course will be different, but pseudo-science is fun).

Super crazy #6 - if you have a large hill near you, park at the top, let it sit cold overnight, push it till you can coast down the hill - does being driven by the wheels make the noise different than driving it (might just be similar to neutral coasting though).

#7 - get a passenger, remove the fuel tank access panel, make them sit near there and listen.
#8 - heavy blankets on all the body you can (poor man's dynamat) - dampen resonance, then try listening.

 

[D.E]

When you changed out the diff, did you change the bearings on the axles? Those things can absolutely HOWL when they go bad.

When driving a steady speed, if you rock the car side to side with the wheel, does the pitch change?

Jordan

I changed the whole axle assembly.

It doesn’t sound like wheel bearings at all, I know what those sound like. More of a humming growl but this is more like gear whine. It does not change when turning left or right. I’ve also repacked and tightened the front wheel bearings.

 

[D.E]

If you take a turn at speed does the sound change? Left and right wheels will go at different rates to some extent. But I guess i'd question whether a bearing would stop howling once warmed up.

Trans fluid - worth checking the fill level to make sure it's properly filled

Now that we know it's intermittent when it does it (but only initially through a drive), then once warm it stops doing it entirely, and is truly only f(speed) that helps narrow down where it might be.

A crazy number 5 would be to mark your transmission output shaft bolts, mark the corresponding driveshaft bolts, separate them (and give enough clearance so they don't hit. Put the car in gear, spin the trans - get your output shaft up to the speeds where it normally presents, see if it still makes noise (lateral/longitudinal loading of course will be different, but pseudo-science is fun).

Super crazy #6 - if you have a large hill near you, park at the top, let it sit cold overnight, push it till you can coast down the hill - does being driven by the wheels make the noise different than driving it (might just be similar to neutral coasting though).

#7 - get a passenger, remove the fuel tank access panel, make them sit near there and listen.
#8 - heavy blankets on all the body you can (poor man's dynamat) - dampen resonance, then try listening.

The trans is actually slightly overfilled (seeps out when I remove the fill plug).

Those are some good tips there, thanks. I’ll try to do some more troubleshooting on a cold morning.

 

[dl242gt]

Drive the car up to speed on jackstands and see if you can hear where it's coming from. Use a friend to go through the gears so you can move around outside the car to listen for the protagonist.