Hook for changing excerpt content when excerpt not set

Title says pretty much all of it. I need a filter that I can use to change excerpt content for those posts that have no excerpt set.

Now, I’ve tried both the_excerpt and get_the_excerpt, they both pass one parameter, and in both cases said parameter is empty string.

That said, I will need to hook onto a filter that has access to the auto-generated except, and lets me change it.