1975 Ford F-100 transmission issues

Truck was running and driving fine till it started to leak fluid I had to go overseas so I let my father and younger brother handle it. They replaced the front main seal then put everything back together. When I got home I started her up and heard a horrible clanking noise like something was hitting the dust cover but cant see anything hitting when I look with a flash light I was just wanting to get an idea of what it could possibly be before I tear her back open any help will not go unappreciated.