07-classic 1500 2whl. dr. 4.3 motor w-5spd. od.

after truck sits for a couple days or more, when i go to move it the clutch will chatter for a few shifts then will clear up. this happens in forward an reverse. if i use it the next day it does not chatter, only when sitting for a few days. i have checked for rear main seal leaks an its dry. once it stops chattering the clutch works fine, no slipping or nothing. only thing i can think of which seems weird is maybe the humidity is affecting it somehow cause it sits outside. i have never even heard of this before, usually when a clutch chatters its done for, but this chatter goes away. thanks in advance