My 1998 Buiok Century has about 86,000 miles on it. On a cold start up, I have noticed that there seems to be what I think is valve rattle noise for about 4 or 5 minutes, then it subsides. Is this normal? Do you think adjustment of the valve lash is called for? Can a do it yourselfer even do that anymore? I'm getting about 20mpg combined, which I think is low. Thank you.