My wife's 500 and my old 88 are the same way. There is nothing wrong with your dad's Maverick. I had mine from 1991 to 2009, and then it went to a friend's son who needed an shotgun for deer season. So far, the only thing my friend's son has complained about with that pls shotgun is that he only has it for about 2 hours each time he hunts. It seems that his dad is like mine was, take the son's lightweight pump to still hunt, leave the heavy azz autoloader with the kid while he sits.
Somethings never change.
Seriously. It is nothing more than a case of Mossberg rattle. the only Mossbergs I have that don't rattle are my old 20 gauge bolt actions. Even my old 200 k has a rattle to it.