1993 Chevy Van losing coolant

193 chev van 6.2 diesel 16months ago the engine was totally rebuilt by a competent localmechanic. In about 4 days of running the coolant overflow blows out about a pint of coolant. The engine does not over heat and runs good. My mechanic chemically checked for cracked heads or head gasket problems. No co2 in coolant. Also checked for air leaks and found none. After running van I can hear gurgling in over flowtank and can see bubbles. We can't find problem. Can you please help