Leave it alone. Or, since it has been sitting, just stick a running water hose in the fill hole & let it run out the bottom for 5-10 minutes. There are loads of cleaning solutions out there, but their use could potentially clean away what was keeping the radiator running "well". New radiator (3 y.o.)? original? What truck?
If you did not cover/seal all openings on the radiator, assume some bugs or other critters might have moved in.
A local shop cleaned & pressure-tested my radiator for a reasonable price. That would be the best option.
Brad
