I had the same trouble with mine, it was a leaking hose that led to low fluid level....I fixed the oil level and it fixed the problem
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1996 2500 12v/nv4500 4x4 335k miles and still going strong!! KDP tabbed, Amsoil 4510 BHAF, evicted silencer ring, plugged wastegate, 5 inch FBE muffled, #100 plate, Isspro boost and pyro, Mickey Thomson ATZ's 285/75R16's, B&W steel flatbed