I have a 90 AWD Astro that has been having this problem for some time. At first it would only "bump" when I gave it too much gas from a stop. Then it progressed to where it did it going up hills. Now it does it while accelerating and it gets more frequent the heavier the engine load is. Up to more than once a second. It feels like it's coming from a wheel but I can't be sure. It's pretty loud and you can also feel it. Any help would be appreciated because I need to fix this problem so I can be confident in taking her out in the snow again!:)

There is a possibility that the AWD trasfer case is rapidly driving and releasing (for lack of better terms) the front driveshaft. While the front driveshaft does constantly "drive" the axle, it does so at a slightly lower rate than the rear driveshaft, or about 0.999:1. The front axle actually over-runs the transfer case under most conditions. There was a TSB issued for this "shudder" condition some years ago regarding the NPG 136 transfer case (GM Bulletin #99-04-21-005b - 09/10/2002). In those vehicles which exhibited this condition, the suggestion was made to fill the transfer case with AutoTrak II oil instead of the original AutoTrak or Dexron III. The "new" oil is a synthetic, and evidently lubricates the sprag clutch packs better to help prevent this. My 2000 AWD has a similar symptom under moderate acceleration on inclines, and the switch to the upgraded oil solved it for me.

Your '90 should have a different transfer case (probably a Borg 1372), but the problem may be similar. If you haven't changed the transfer case oil in a while, it may be time to try that.

There is a possibility that the AWD trasfer case is rapidly driving and releasing (for lack of better terms) the front driveshaft. While the front driveshaft does constantly "drive" the axle, it does so at a slightly lower rate than the rear driveshaft, or about 0.999:1. The front axle actually over-runs the transfer case under most conditions. There was a TSB issued for this "shudder" condition some years ago regarding the NPG 136 transfer case (GM Bulletin #99-04-21-005b - 09/10/2002). In those vehicles which exhibited this condition, the suggestion was made to fill the transfer case with AutoTrak II oil instead of the original AutoTrak or Dexron III. The "new" oil is a synthetic, and evidently lubricates the sprag clutch packs better to help prevent this. My 2000 AWD has a similar symptom under moderate acceleration on inclines, and the switch to the upgraded oil solved it for me.

Your '90 should have a different transfer case (probably a Borg 1372), but the problem may be similar. If you haven't changed the transfer case oil in a while, it may be time to try that.

Thanks man, I'll try that. I don't suppose having one slightly larger wheel from a Caprice could have anything to do with it?:grinno:

I know it didn't used to do it at all.

You have one wheel that is taller/shorter than the other 3? Yes that could cause a problem....

It always helps to include all the details. Yes, as CD wrote, one tire/wheel of a different size is going to be a real problem on an AWD.

It always helps to include all the details. Yes, as CD wrote, one tire/wheel of a different size is going to be a real problem on an AWD.
You think it might have caused permanent damage to the transfer case?

What is the wheel size on the astro? No junkyard around here seems to have astro wheels. It's been a while though so maybe they do now.

Your OTHER three wheels should be the correct size, and possibly even the spare. Match those, and the tire size so the outside diameters of hte tires are all the same.

As for whether there has been any permanent damage, it is impossible to predict, and won't be known until all the tires are the same. Consider looking for an AWD/4WD Blazer, Jimmy, Bravada, etcetera as a source for wheels. A lot of those were equipped with an externally mounted, full-sized spare.