This hole could cause a leak in the chimney that would appear a lot like a roof leak suggests having a Mason attend this. Litespeed Construction can also backfill this crack in a hole with silicone. A mason is not available.

This chimney is in a unique position because it is a valley where water can pond behind it. The best recommendation is to put a membrane behind the chimney if water ever stands. A cricket cure would not necessarily solve the problem. It might be going to cause worse ponding at this chimney.