come knocking on the door

To present (itself) as a possibility or (usually desirable) opportunity in the near future, especially temporarily. It was hard leaving my parents and all my friends from high school, but when an opportunity to attend school in Europe came knocking on the door, I knew I had to seize the chance. What would you do if the chance to make half a million dollars suddenly came knocking on the door? You have to be ready to act when opportunity comes knocking on the door, or you might end up regretting it forever.