Diagnosing noise/hum

[cwest]

86' K20.  I've got maybe a couple hundred miles on a fresh rebuilt SM465.  Rest of truck normal mileage for it's age, with rebuilt 350 about 60K ago.  I'm starting to notice a hum, almost like the sound mud tires make on pavement, but I have street tires.  It starts at about 10-15 MPH, and continues to rise in pitch until its almost inaudible around 65MPH.  From the cockpit it sounds like its coming from the front end, area, but it's really hard to tell.  The hubs aren't locked so nothing is turning with the front end, I've felt the brakes after long trips and they're cool to the touch, nothing in the engine bay seems to be overly hot or out of sorts.  This is my first truck of this model, so I'm not sure what noises are just personality, and what noises I need to worry about.  It's def drive train as it increases with speed.  Any thoughts?

[pneuner750]

I had the same problem! The throw out bearing was shot. I had the clutch replaced, trans rebuilt. Mine is not four wheel drive. I Hope this might help.

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[Irish_Alley]

does it change when you press in the clutch?

[pneuner750]

Definitely! Half way in it sounded like it was going to blow up.  It never gave any warning.  The truck barely made it to the shop.  I live in Phoenix,  I hope this helps.  Is it possible the transfer case is shot?

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[Irish_Alley]

does sound like a clutch/bearing issue

[Don5]

It is probably the throwout bearing. It might also be the pilot bushing spinning in the flywheel too. If it wasn't replaced or a used one put back in, it will spin until it heats up. It will do this until one day it will expand and catch. The noise will stop then. If it is a throwout bearing, the noise will never stop. If you take it apart replace everything then you will know that it has been done correctly.

[cwest]

Well, the hum went away for a while, then I noticed it come back on a long freeway trip when I was going around an on ramp, so I'm thinking wheel bearing or something front suspension.  Nothing seems to be heating up, so I'll just keep and eye on it.  However.. All this pilot bearing talk I think caused a pilot bearing issue:)  I am now getting a chirp from that region.  The chirp happens when the clutch pedal is completely out, I can then put the smallest amount of pressure on the clutch pedal and it goes away.  I'm thinking the forks on the pressure plate are just barely touching the pilot bearing and causing a little chirp when disengaged.  Does this sound like the case or is my pilot bearing actually going bad after 500ish miles?  And is there a way to adjust the forks and pilot bearing away form each other by a milliliter?