2001 LeSabre Noise upon start up

I just purchased a 2001 LeSabre with 43K. I have had the car for 2 days now and it has now started making a "sounding like rocks in the engine" upon start up. After 3 or so min the noise goes away. When I turn the car off and let it sit for more than 3 hours the noise is back. When I turn the car off and let it sit for less than 3 hours I have no noise. Is this something I need to be concerned about? Could it be the timing chain or the harmonic balance? I need to resolve this problem soon. The dealership told me I can return the car within 5 days if I do not like it or have a problem with it. I also have a 30 day we will make it right guarantee at no charge to me. I like the car a lot & I would like to keep it, but if I have a problem on my hands I need to know now.

Thank you for your reply

I would be concerned about it, take the car back. I cannot see this situation getting any better. It could be a main or rod bearing that is on it's way out. Or it could be something simple, but why take the chance, have the dealer look at it.

Well I did just that. I took it back and it ended up being the exhaust system. Something had come loose in the converter. After a day in the repair shop at the cost of the dealer I now have a new converter along with a few other new parts at no charge to me :)