I own a 1998 Subaru Legacy wagon with the 2.2 liter motor, automatic. Recently I noticed a slight engine knock/tap sound mainly first 10 minutesof driving when driving aound town. Not major loud at all but you can here it up front slightly. Disappears after car warms up. I read somewhere this is a common issue for this model and may not effect engine wear. It has 136,000 troublefree miles on it. Any information appreciated and if others have this issue.