I have a 94 4.0 and I am getting a very loud tapping from the valve cover area whenever I first start it up. It lasts anywhere from 1 to 5 minutes and is tune with the RPMs. After a bit, it instantly just quits. It seems to me like I have a sticking valve. Any suggestions such as Seafoam, Marvel Mystery Oil, Dura Lube, etc? Should I not drive it the 1500 miles I was going to this next week, or is it really not that big of a deal for a motor with 140k miles? TIA, JP