Humm at highway speeds

Discussion in 'GM Powertrain' started by johnsonjf, Jul 23, 2010.

  1. johnsonjf

    johnsonjf New Member

    Good Morning All,

    I have a 1999 GMC K1500 Suburban SLE 4x4. The transmission is a 4L60E and the transfer case is a NP8. The transfer case has 2 small holes in the back side, close to the area of the internal pump. I have patched the holes from the outside, AutoTrak II is expensive, and they haven't leaked.

    Driving down the highway between 60-70 I get a hmm from the underside, doesn't sound like tires, and once I get past 70 I can lift the accelerator and then get back on it and the hmm goes away until I let off the accelerator again then the hmm returns until I get below 60. At speeds below 60 I can still hear the hmm once I let off the accelerator.

    I am thinking it could be noise from the transfer case pump but not sure. Would it be easier and cheaper to replace just the pump or should I replace the entire case?

  2. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Take a look at your U-joints as well. Also take a look at the rear carrier since that can humm as well.
  3. hundojoe

    hundojoe Rockstar 100 Posts

    Could be the bearings in rear diff, or the axle bearings. Also not uncommon for the front diff bearings and gears to go from not being used. I doubt its the u-joints as they tend to make a clicking sound when they go bad.
  4. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    Depending on how bad they are. I have had them humm at highway speeds as described.

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