Motor problem ford van

My Son inLaw has a v/8 ford van that has developed a loud internal exhaust problem like one cylinder missing loudly. What could be the problem?

What is an “internal” exhaust problem vs an ordinary one?

It would help a great deal if you would tell us what model and year you are talking about and the general condition of the it as well as any repair history.

When was the last “tune-up” done?

It probably is an exhaust leak. It could be a broken gasket at the head; a cracked exhaust manifold; a loose header pipe; a broken gasket at a header pipe; or a holed header pipe. You should have a mechanic look at. He/she should be able to pinpoint the leak and give you an estimate on the cost of replacement or repair.