I have had my 2003 Explorer for the past year and a few months and have had a knocking noise from time to time when I start the engine. It was very light at first and barely noticable until this winter when it got a bit louder. At that time it would go away fairly quickly tho (10-20 secs). Now this summer it has been knocking much louder and continues to knock for a minute or up to 5 mins. I took it by a Ford dealer just to get a quick opinion to see if it was anything that needed attention and they said it was my lifters and that this happens with Explorers as you put mileage on them. Mine has 99,000 miles. They told me if I thickened my oil from the recommended 5W-20 to a 10W or 20W that it would stop the knocking noise. I changed it to 20W-50 and it completely stopped for a few days but the knocking noise is back now. It is very light but it is there again and stays for 30 secs to a minute or two. Can anyone give me an idea of what is going on or let me know if you have had this problem as well. I want to make sure this isn't serious and I am treating this properly. Thanks for your help.