Yesterday I noticed a relatively loud tapping noise from the front valve train area. It sounds loudest near the two coolant sensors next to the bottom of the valve cover. I had the valves adjusted two months ago. Mileage is 92k. A couple of years ago I had loose exhaust nuts that made it sound like an old Volkswagen. Can a loose exhaust nut sound like that? Possible timing chain issue or bad valve? Any ideas appreciated. Thanks in advance.