Leak from chimney Cleanout box

[Victory82]

Water leaks out of the clean out box in my basement when it rains.. a purposely left the garden hose on next to the chimney to test it and about 10 minutes later their was a small puddle in my basement so I know it’s not because of the roof or anything.. the problem is bellow grade. I’m currently digging outside and so far the chimney looks fine ( I’m 3-4 feet down ) .., Should I keep digging or or fill the hole with water and see if it leaks ? The box is filled with debris