why is there white sap coming out of my tree it's a globe willow

Could be beetle grubs tunneling the sapwood layer of the tree causing a botanical hemmorhage of the trees sap through the bark. If it is this, perhaps other members familiar with this particular tree will tell you more about how to control this problem if it needs control.

I don't know if you're in the UK or the USA - if the latter, there are more possibilities as to the cause, but what you are describing sounds like slime flux which hasn't yet started to get smelly. This particular tree is very susceptible to this problem - all you can do is keep the tree well watered. if the smell becomes really noxious, you can wash down the trunk where the problem is with a 5% bleach in water solution, but you will need to repeat this treatment - the organisms which invade the leaking sap cause the smell, and they'll return.

The tree might recover, or it might get worse and worse and start to look terrible, at which point, best remove it and choose a slower growing tree, less likely to suffer this trouble.