I have a 1997 Audi a8 that I bought a week or so ago, and took it to the mechanic to check it out. He then found an oil leak on the rear main valve and a warped head on one of the pistons. He enlarged the distance and tightend head(how he explained it).the car now makes an odd clicking noise almost the sound of a diseal what could this be and what problem could this create in the future? Btw car has 160k miles on it