I am going to imply that I have a feeling which doesn't let me believe something can be absolutely true (I have some doubts in my mind whether it is true or still I should not believe it completely.) How can I convey this message?
I know two idioms:
First: Deep down inside
Second: in one's stomach (I have just heard this one, but cannot find any reliable source for it.)
Let me make up a scenario to clarify my intention.
I am wondering which one of these two idioms can be used in scenario below?
Though he told me that he would pay me back the money he borrowed three months ago, but...........
a. deep down inside, I don't trust him. Because I know that he's unemployed at the moment.
b. I don't trust him in my stomach. Because I know that he's unemployed at the moment.
To me, 'a' makes a perfect sense, but I doubt if it is natural here.