Exhaust manifold/passenger side

Have a 1998 Ford Explorer XLT v-8, heard a "pop" then what sounded like a cyclinder compressing air out of a hole, like a spark plug had let go. Not the case. I examined the exhaust manifold and saw a clean bore hole that might have housed a sensor of some sort. Q: Can this be repaired, what goes in there and will it be expensive? :(