86 GMC S15 4x4, 2.8, 5 speed, 280,000km. (around 175,000 miles)

Owned it for 5 years, was stolen and the engine blown before I bought
it, so I don't know how old the clutch is. (don't know if the place
that swapped the engine did the clutch or not.)

I have a teeny hydraulic leak in the clutch somewhere - just a topup
about once a year... it's a winter truck. Sometimes in rush hour the
clutch pedal goes soft and you have to "pump it up..." which I was
figuring was the leak and/or the clutch just getting worn...

Anyway, new noise - a whump or a chuffing noise when shifting or
stopping. Thought it sounded like the clutch grabbing as it came up to
speed or something in the transfer case (it's noisy.)

What I've found is if I shift without using the clutch it doesn't make
the noise. If I shift slowly and into neutral between shifts and barely
touch the clutch it doesn't do it.

Is it possible for the throwout bearing to "vibrate" like a buzzing
sound? A clutch job would probably double the value of the truck, and
I'm hoping to milk one more winter out of it, but I really don't want to
totally destroy the thing either...

The clutch doesn't slip under hard acceleration, but with slippery roads
and a 2.8 there's no such thing as hard acceleration anyway. Sometimes
when stopping it'll do the same thing if you press the clutch down -
like it's not disengaging all the way - if you pop it into neutral the
noise stops.

I'm thinking throwout bearing is worn, or the slave cylinder is leaking
and not extending far enough and/or the clutch itself is worn.

Opinions? It's -30, so yanking the tranny out for a quick peek is
notgonnahappen... If it dies I go back to driving the wife's car until
spring.