My 12 yr old Lab got out of the yard yesterday, so I jumped in the 97 Cougar (164400 miles) to check the perimeter of the neighborhood before we started searching house to house. I ran it kind of hard for a couple of miles, went back to the neighborhood and pulled alongside my daughter, who was dropping by. We sat there for a couple of minutes and she said, "what is that noise?". I got out, didn't hear anything, then a clatter arose that lasted for a few seconds, then went away. Drove back to the house, and let it idle and the noise would come and go. Let it sit overnight, then cranked it this morning. No noise for a while, then it started the intermittent clatter. Put a vacuum cleaner tube against the RH valve cover and its definitely valve train related. Could this be a lash adjuster getting ready to go? Doesn't seem to follow rpm, just intermittent at this point. Any ideas? Thanks in advance.