Hi, I saw this on TV in the very late 50s or early 60s - so it was in one of those 'cheap movies for TV packages' like what the MST3K guys started with. So, it won't be famous. It would have to be really cheap or semi cheap and kinda old to be in those. This means it _might_ be from the 40s

Old house, hidden treasure, trapdoors, an underground river, etc. Lots of movies with these elements are out there.many, many of them.

The one thing that stands out strongly in my memory about this one is the end: once all the clues are deciphered you are led to a fireplace and mantle that has a Ship's clock - with spokes - over it. The clock's spokes are revolved like a combination clock and when it is done correctly: first a small trapdoor in the ceiling opens up and a little bell Dings, and when you look up a trapdoor beneath your feet opens up, and you drop straight down into the raging river never to be seen again.
The hero figures out the state of mind of the treasure hider and steps back when the little bell rings. The very last scene is his sidekick comic relief holding onto the spokes, hanging over the hole and yelling for help!

This _may_ be a piece of junk w/ just this cool twist. I do not know. It has 'haunted' me for decades. 8-)

Thanks for any help.