Hi - been awhile since I've been here - the old 98 has been running fairly well up until now...

However, my wife has been complaining about some rear end noise, so I took it out for a spin last night on the highway. If I bring it up to about 115km/h and then take my foot off the gas, in about a second it starts to make a rumble/roaring sound in the rear end. If you put your foot back on the gas, it stops. Also, I took it down a long hill and got it to do the same thing - when I took it out of D and put it in N the noise continued... Not sure what this is.

I haven't had much time to get under it and have a look, and I hate going into a mechanic unless I have a sense of what the problem is first, to make sure I don't get some line of crap. My initial thoughts were brakes or differential, but because the noise goes away when I have my foot on the pedal that is throwing me.

Any ideas?

Thanks!

 

I would definitely say it is in the rear drivetrain. My first thoughts would be the universal joints, but it would not hurt to service the rear diff and have a look at the condition of the gears.

 

Thanks - I talked to the wife over lunch who told me it clunked going into reverse today, so I'm guessing the Ujoints are likely the culprit. But would that explain the roaring when you take your foot off the gas? Why would the noise go away when you put your foot back on the gas?

Cheers,
Grant

 

U-joints can do some funny things. It is all about how the u-joint responds to the change in torque. When you are on the gas, the driveshaft is pushing the rear axle. But when coasting, the engine is actually slowing down the vehicle so the direction of torque applied to the rear driveshaft changes.

 

Good point, although it still roars when it's out of gear - I'll get under it tonight and check for play. Thanks for the info - I'll report back when I figure it out.

 

My point on the change in torque still applies when in neutral as well. The driveshaft could shift in the u-joint when not being driven by the transmission, causing the noise/vibration.

 

Just wanted to report back - it was the u-joint. I had a very small amount of fluid that had leaked from the diff, but it's pretty minor. I did a rear brake job on the weekend (rotors, pads) and the u-joints are getting done today while I'm at work.

As well, found that the oil cooler lines are leaking - bloody hell, does GM ever charge a lot for those two lines ($400 CDN)!! Jobber parts from the mechanic is $170, and on RockAuto is about $55 (plus shipping). Given the length of time these things last (GM or otherwise) I went with the jobbers. I understand that they are a bit of a pain to replace, so I just had the mechanic do it today.

Just wanted to say thanks for the input!!