Knocking from suspension, 1996 Chevy K2500?

When I first fire it up and set off, I can't hear or feel anything at all, no matter what the speed. However, after about 10 miles maybe, if I slow down I can hear a knocking, coming from presumably the wheels, that keeps time with the speed of the wheels spinning. (Drive faster, faster knocking, up until about 40mph, then I can't hear it.)
Is it the wheel bearings going out? If so, why are they not making noise all the time?
I've tried jacking the truck up and spinning the wheels by hand, but they didn't make any knocking noise, but they did make some faint noise in reverse?
Ideas?
 
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