Linden leaf curling
Is there any damage to the tree trunk or signs of insects on the tree such as mites or aphids?
There are no insects noticeable.
Is there a sticky substance or shine to the leaves at all?
No sticky/shiny residue
The most common linden tree diseases are listed at the following website with information on what to look for. Be sure the tree is getting enough water. Curling leaves on plants can mean lack of moisture. See: https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/linden/leaveswilt.html