Dan,
You've got a lifter bleeding down, which causes the "clacking" until it
pumps itself back up.
My 85 GL does it all the time, if I leave it sit for more than 2 days.
It usually takes about 5 miles to return to normal and will stay that
way until I leave it sit again.
Just another project I have to get to             ( changing lifters )
The  only thing that might happen is possibly the push rod might jump
off the rocker, but I doubt it.

Happy Vanagoniting,

Terry