Howling sound when let off gas?

[markmitchinnh]

I just traded van for van to get a 1994 safari when driving it for the first time I noticed this sound possibly from the rear of the vehicle but only when I let off the gas pedal. Any ideas? It is a load howling sound.

[LiftedAWDAstro]

Pinion bearing in the rear axle most likely. Check to see if the pinion is loose. If it is you can try tightening it up to get by for some more miles. Might be time to just swap the axle out or throw new bearings in.

[markmitchinnh]

OK Thank you, I am doing a full tune up including trans filter and rear gear oil so that's a good time to do that. I have another rear axle assembly from a 92 but its got 3:42 gears I don't know what this one has at this time.

[kc5gxl]

OK Thank you, I am doing a full tune up including trans filter and rear gear oil so that's a good time to do that. I have another rear axle assembly from a 92 but its got 3:42 gears I don't know what this one has at this time.

If you still have the glove box and ID sticker in it, you can tell what the ratio is. The rear end ratios are a 3 digit alpha-numeric code. I believe the 3.42 is GU?, but I may be mistaken. I know that somewhere on this site, hidden in all the millions of files here is a run down of all the axle codes. Just can't think of where it is at the moment. I am sure someone can help you get there, though.

I also believe it is the rear end howling. It may be the pinion bearings, or maybe the ring/pinion gear mesh up.

Drive safe,

Dan Harriman
91 Safari Shorty V8 RIP

[GnarliSafari]

gu6 would be 3:42,all rpos are 3 digits, gs, gt, gu, gv and gw codes followed by a number indicate your ratios normally.

[Cobra]

taken from viewtopic.php?f=10&t=22

GT4 Rear Axle: 3.73 Ratio
GT5 Rear Axle: 4:10 Ratio
GU5 Rear Axle: 3.23 Ratio
GU6 Rear Axle: 3.42 Ratio

[MountainManJoe]

I don't think Astros ever had 4.10's stock did they?

[LiftedAWDAstro]

They were optional up into the 1990 or 1991 model year. They were standard in the early 2.5L 4 cylinder models.

[markmitchinnh]

I changed the fluid and checked everything I could and found nothing wrong. However I noticed the transmission seems to buck a little when the noise is present, could this be the lock up converter? and the antilock brake light is on in the dash.

[GnarliSafari]

those same rpo's are used in other gm vehicles also, some vary depending on age, but most are the same. for instance my truck also has the GT4 G80 (3:73/posi) combo and it obviously has a much larger rear-end than my van has, 11.5" i believe.

[kc5gxl]

I changed the fluid and checked everything I could and found nothing wrong. However I noticed the transmission seems to buck a little when the noise is present, could this be the lock up converter? and the antilock brake light is on in the dash.

Hello Mark;

Usually if the torque converter is going out, you will either notice the engine starting to run a bit faster, especially when going up a hill (not because of the hill). Maybe a couple of hundred RPM, because the converter is slipping. Letting up on the gas pedal will usually get the engine back to the normal RPM...for a while.

Secondly, if a converter is starting to stick, the vehicle will stall whenever it is put in gear, but will run fine in park or neutral. One way to check if a converter is working is to drive done a level road, at a steady speed, and while maintaining the speed, lightly put your foot on the brake pedal. Not enough to slow the van, but just enough to light the brake lights. You should notice an increase in RPM. When you let up on the pedal, the RPM will go back to what it was before the test.

By the description of the noise you gave us yesterday, it sounds like the diff is out of specs. The way you find out is by taking the rear cover off and set up a dial indicator so you can see how much backlash you have on the ring gear. If the backlash is within specs (usually no more than 0.008") you clean the ring gear teeth and put Prussian Blue on the teeth. While maintaining a slight bit of tension on the ring gear you then rotate the pinion gear so the ring gear makes one complete revolution in both directions so the ring has made contact with the pinion where to see the tooth pattern. You compare this to a template to determine if the pinion has the correct depth.

The backlash is almost imperceptible to the naked eye without a dial indicator, and the tooth pattern is non evident without Prussian Blue...or some such...to read the tooth pattern.

If the front of the differential is leaking some, that is a good indication the pinion bearing is wearing out. That can have an effect on noises.

Unfortunately, unless one has up close and personal knowledge of a used vehicle we may be interested in buying, we have no idea of what was wrong, or right, with it before we purchase it. All we have to go on is what we are told, and what we may catch on a road test. It is kind of a 'Caveat Emptor' when it comes to buying a used car. Who knows, the noise may be one of the reasons the previous owner wanted to sell it.

Good luck with your diagnosis, and please keep us informed with what you find.

Drive safe,

Dan Harriman
Orange Texas

[markmitchinnh]

One of my neighbors has a 2001 GMC Safari it needs a trans and a drivers door, I offered to buy it for parts and he said he could haul it across the scales for $350.00 so to me its not worth it as I only need the seats, carpet, rear axle & factory chrome wheels. So I went back today and told him i'd give him $150.00 for the parts I need and give him mine in return.
He not only accepted but said I could do it there and use his tools. I have no idea what the gear ratio is in the 2001 but mine is 3:42 and as long as its that or higher I am fine with it, even a 3:08 would be good on gas.