91 Silverado Clutch Chatter...

This truck has 45,000 miles on it and i bought it new in 1991.
Everytime time I start it the first time for the day I get a lot of clutch chatter which really shakes the truck,after the clutch has been used a few times on my way to work, it stops and everything is good until the next morning I take off in it.
It only chatters in first gear from a dead stop, all other gears are smooth.
What could it be? :uhoh:

I would have to say it's the Throw Out Bearing. I thought they chatter all the time so don't qoute me on any of this but that would be my guess.