Weird noise in cam....

i have a 2000 Ford Contour SE DOHC 24V V6. It has about 93,000 miles on it and it runs pretty good except for a loud plunking/blooping noise that comes from the cams, you can heir it even louder when you take off the oil fill cap. What is causing that, how do i fix it??? My mom did dive it too with low oil for a while.