Lifter noise when start up

I would make sure that your Intake Manifold Gasket is not leaking coolant into your oil. You won't be able to see it at first, but it is a common problem on midsize GM cars with a V6. Check the color of your coolant. Is it clear and Orange or is there some discoloration from a little bit of oil? Is the coolant getting dark brown or have brown stuff floating in it? Best of Luck!