Hey I have a 1998 K1500 Ext. Cab 4x4 with Vortec 350 and A 5 speed Manual Trans which I believe is the NV3500 the truck has 157,000 miles on it. There is a humming noise coming from the tranny while Im driving and while my speed increases does nothing with RPM just speed, The tranny still operates perfectly and shifts fine and I feel no slack and no grinding or binding in it Just the Humming noise and I can get going take it out of gear and let out of the clutch and it goes away then comes back when I put it back in gear. I was wondering if it could just be low on fluid I have not checked it yet but Have parked it for the time being till I can get some info. Like I said there is only a humming noise and it is in every gear but still performs like a new one. Any help I would really be thankful cause I really hope I don't have to put in a new tranny I just got the truck a couple weeks ago and It was so hard to find a nice ext. cab 4x4 with a 5 speed. Thanks in advance