The Unknown (1946) Henry Levin

Twenty years before we start, Rachel Martin is about to leave her large family home in Kentucky with her husband of some months, but her parents don't know and are happy to ignore the whole thing and have her marry someone else. A struggle over a gun brings the accidental death of her father and the family bricks him up behind the fireplace, as led by her mother Phoebe, who is as domineering a bitch of a family matriarch as you can imagine.

Back in the present day, Rachel's daughter Nina Arnold visits the house for the first time, for the reading of her grandmother's will. She's never met her mother and her two uncles, and of course the house is not a happy one. Rachel is insane, Edward is a deaf sculptor and Ralph is an alcoholic. Luckily for Nina, she's brought along Jack Packard and Doc Long to accompany her, and they're just the folks to find their way through the shenanigans that soon begin, given that Nina must stay the night in order to be eligible for whatever might be left to her.

What we have here fits less with the previous two films and more with what I expect the original radio series to be. Its status as a serious influence on Scooby Doo is far more obvious too, with all sorts of bizarre activities going on that would be highly appropriate for the gang in the Mystery Machine to investigate: no ghosts per se, but secret panels, locked doors, crypts, the works. Of course, there's no gang, just Jack Packard and Doc Long, but that's enough to crack this case.