If your 4WD GMT800 has a rattle or grinding noise at highway speeds that goes away when you shift to 2WD or 4HI, there’s a good chance your NP246 transfer case has the pump rub issue. This is a very well-known failure on these trucks.
What Is Pump Rub?
The NP246 transfer case uses a plastic oil pump inside the case. Over time, the pump and its housing can wear against the chain or internal components, creating metal-on-metal contact. The result is swarf (metal shavings) in the fluid, accelerated wear, and eventually transfer case failure if not addressed.
Common symptoms:
- Rattling or grinding noise at highway speeds (typically 55–75 mph)
- Noise changes or disappears in 4WD
- Gray or silver metallic fluid when you drain the case
- 4WD engagement issues (in advanced cases)
The Fix – Pump Rub Repair Kit
GM released a TSB and Dorman / GM both produce repair kits. The repair involves replacing the plastic oil pump assembly with an improved version.
Part you need:
- Dorman 600-911 – Transfer Case Oil Pump Repair Kit — fits most 1999–2007 GM trucks with NP246
The kit includes the updated pump, updated pickup tube, and hardware.
What the Job Involves
The transfer case needs to come out of the truck and the case needs to be opened. If you’ve done transmission work before, this is similar in scope.
Basic steps:
- Drain the transfer case fluid
- Disconnect front and rear driveshafts
- Remove transfer case from transmission (6 bolts on most applications)
- Split the case halves
- Remove worn pump assembly
- Install new pump and pickup tube from the kit
- Inspect chain and sprockets for wear — replace if pitting or stretch is present
- Reassemble, reinstall, refill
Fluid spec: Use Autotrac II (GM spec) transfer case fluid. Do not use generic ATF — it will cause shudder and premature clutch wear.
Check the Fluid First
Before pulling the case, drain the fluid and inspect it. Gray or silver metallic fluid with visible particles = internal wear is already happening. At that point the pump kit is the minimum — you may also need a chain and sprocket set.
Clean fluid with only a mild rattle = the repair kit alone usually stops progression.
Rebuild vs. Replace?
A used NP246 from the junkyard runs $100–200 but you don’t know its history. A pump rub kit on your own case is typically $60–90 and keeps your known-history unit going. If the chain is worn too, full rebuild kits are available.
Drop your symptoms, year, and mileage below. Happy to help confirm if this is your issue before you start pulling things apart.