Recently after a long trip, my 99 Expedition (75,000 miles)developed a tick, which sounded as if it were low on oil (it was not low). After several days of driving, I started it and it sounded as if it were backfiring repeatedly. My mechanic tells me the number 5 spark plug had exploded and damaged the header which will now need to be replaced at around $4000.00. He says I could put a rebuilt engine in at around $6000.00. I still owe around $11,000.00 on the vehicle. Has anyone heard of anything like this happening before? Any suggestions on how to proceed?