It sounds like a valve noise. What you can do to verify that it is in the engine or not is remove the drive belt and start the engine. If the noise is still there then it is in the engine or exhaust, if not then is is related to a driven pulley. Lots of vehicles with 100k miles have some valve noise. I would check the oil and make sure it is not diluted with coolant. If it is the intake is leaking. It could also be an exhaust leak that seals when the vehicle gets warm. You cant say just by listening that the engine will go bad , obviously it wont fix itself but it can still last a long time.
I am sure the nois is inside of the engine.
Can you explain me more detailed what signs of coolan leak (what i will see in oil and how to check the oil? should i remove oil for check?) I know Buicks have this problem and i have to be prepared to diagnose it in the future.
The oil will look kind of like chocolate milk. You can see this on the dipstick. Sometimes you will see a milky sludge on the bottom of the oil cap. Sometimes you can see the oil level on the dipstick is abnormally high due to coolant in the oil. No need to drain the oil to tell. Best time to check is before changing the oil. Hope this helps.
Thank you very much for the information! It is very helpfull!
I have another question which is very important for me.
I am an international student of the local University and I need a reliable car for the next 5 years (period of my education here). Local dealer offered me this Buick Century (here is the link: www.brookingsautomall.com/VehicleDetails/497560603) I inspected this car, everything works, condition is good, runs good, only engine noise mentioned above and oil spots on the engine where found. They assert that this car is reliable enouth and appropriate to my needs. And it would be very interesting for me to know your opinion about it. Is it realy reliable and meets my requirements? Is price reasonable? What to expect if i buy it?