Why does your 2000 Grand Am GT engine chatter soounding like some serious lifter noise when cold for about 3 minutes until it warms up Once it warms up it goes away Will this hurt anything?

you need to let the engine warm up so the oil and other lubricants will protect the moving parts of your engine and tranmission ***The 3400 V6 is, and always has been poorly manufactured - The noise you are getting is due to loose/incorrect factory internal engine clearances. There is nothing you can do about this problem besides use a small amount of oil stabilizer, like "Lucas oil additive". There is good news, though - As long as you change your oil regularly, this problem should not get worse, and therefore is more of an annoyance than anything. One thing you should know, if your car uses any coolant at all (if you have to add coolant more than once a year) then you need to have your car inspected for leaks in the intake manifold and head gaskets - These repairs are very common (ALL 3400's will have to have their intake gaskets replaced by ~100,000 miles) and can result in coolant getting in to your oil. Coolant in your oil will do permanent damage to your crankshaft bearings, and can also cause galling of the sidewalls of your pistons. Both of these problems will cause a severe engine knock (like a hammer on the bottom of your engine) and will ultimately result in catastrophic failure.

Very true. My 3.4 turned 100,000 miles and about a month later the head gasket and intake gasket both blew. My car also had this noise, but like mentioned before if you maintain regular oil changes it shouldn't get worse.