2005 Mitsubishi Galant Engine makes pinging noise when cold

I have a 2005 Galant GT V6 with 160,000 miles. The problem I been having is when the engine is cold the engine will ping very loudly. So, loud that is sounds like it is going to explode. After a minute the engine warms up and the enginer sounds fine. I have change to synthetic oil and the recommended weight, but has not solve the problem. I took it to a mechanic and at first thought the lifters needed to be adjusted and later told me they were self adjusting. The only thing I can think of is the oil pump may be going out or the car engine will need to be overhauled. I was hoping to get another 40,000 miles as this is a great car. I drive an average of 650 miles a week so I change the oil regularly.