I'm not a gardener by nature, whereas my wife is, but our willow tree
has what looks like a load of dried sap coming out of a spot on the
trunk about 4 feet off the ground, forming a small solid waterfall
effect, and there are several ants crawling around it. The waterfall is
about 4 inches in height.

Maybe it's an internal ant nest? If so, is it bad for the tree, and how
to treat it if it is?
Otherwise, I can't find what it is, so any help welcome.

Is there a good website to ID such problems? All that I have looked at
haven't helped.

--
Davey.