I recently went thru a similar situation. In a nutshell, my great uncle left me the property his hidden and not so hidden stashes were located on. For years he had told me things he told no one else. He never really came out and showed me his stashes but would always make comments like this or that place would be a great place to hide something. The house had 2 safes. Both combinations were my birthdate. It was pretty clear after the fact, that he wanted me to find everything. A letter, and some other documents in one safe confirmed this. I really think that you were told for a reason as I was, and you should absolutely feel no guilt. Having a 4 year court fight with greedy relatives helped me get over any guilty feelings I had in a hurry....
Something else you can ponder further. My mother was also left a house. My great uncle told her the same types of things about her house. Neither of us ever knew what the other had been told, and though we are very close, we never talked about it with each other. Maybe there is more than one stash on your grandfathers property. Maybe you have another relative who knows the where-abouts of that stash. If your grandfather thought anything like my great uncle, having just one stash is very very unlikely.......
I also agree with the others keep your mouth shut.