1994 Chevy Blazer 1994 Chevy S-10 Blazer noise in front end

Our 1994 Chevy S10 Blazer has a grinding and/or whining noise in the front end (it seems to come from the passenger side). This happens only sporadically, usually after the truck has been driven over 30 minutes or more. After it happens, there's a clunk as if something is breaking free, and then it operates normally again. The rpms don't go up. The steering is not affected. Braking doesn't affect it, although slowing down seems to lessen the high-pitched whining and grinding somewhat until it finally clunks and the noise stops. We've had the truck looked at by four different automative shops, one specializing in 4-wheel drive problems. No one can figure out what's wrong with it.