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NP241C Grinding/Rattling Noise

790 Views 0 Replies 1 Participant Last post by  TexAg Troop
I have a grinding noise which I'm 99% sure it's coming from my transfer case. It happens when I'm coasting or lightly accelerating, and it's very obvious above 25mph. Here's some of the info:
  • NP241C from a 1990 Suburban (passenger drop)
  • 27 spline input
  • SYE kit from JB
  • mated to a 4L60E
  • filled with the AC Delco AutoTrac (blue stuff)
  • front and rear driveshafts from Tom Woods (very recent)
  • this isn't my daily driver but the noise has just started recently

And what I've done already to try and figure this out:
  • rebuilt with all new bearing and seals
  • replaced rear output bearing with a new bearing (John at JB was very helpful)
  • replaced pump

I tested the truck on jack stands and:
  • transfer case in N it doesn't make noise, or at least anything noticeable
  • 2wd makes noise
  • 4wd makes noise with both driveshafts attached, or front driveshaft only, or rear driveshaft only

It has made the same noise before and after all of the replacement parts so the only moving parts left are the planetary and the chain, and since it happens in 2wd with the chain not moving I could rule out the chain (it looks fine with no sag).

Any help is appreciated and I'm looking to buy a new planetary, but it seems fine, there isn't any play in the pinion bearings although there is up-down play.

Here's a video of the noise:
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