Thank you for your question.

Linden trees can get a leaf gall that is caused by an eriophiyid mite. Without seeing a picture of the leaf and cannot say for sure this is what is happening to your tree.

Here is a Fact Sheet on Insect and Mite Galls:

https://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/

The following is from the Fact Sheet:

"Eriophyid or gall mites make a variety of plant galls. Small finger-like galls, pocket galls, or felty masses of plant hairs (erinea) on leaf surfaces are common types produced by these mites. Irregular growths on flowers or buds of ash and cottonwood are produced by other species of gall mites. Some gall mites that feed on top of leaves also produce irregular leaf curls..."

Most galls start in the late spring and early summer when adult insects become active and start to lay eggs. Usually the galls and the insects that form them do not do significant damage to the tree. The galls are conspicuous and unattractive. Once galls start to form there is little you can do to stop them. If this occurs over several seasons, the growth rate for the tree may be slower and the unattractive look will continue. Keep your tree watered adequately during the whole year, watering when we have dry times without significant moisture even in fall and winter. 

Eriophyid mites may be controlled with dormant oils. Insecticidal control can be difficult because the timing is crucial. Sprays must be done during the time when the insects are laying eggs. During this time, CSU recommends using 'Sevin' or permethrin as insecticides for control of eriophyid mites. Because the insects have already laid their eggs for this year, it may be too late to do anything about the insect. 

Here is a Fact Sheet on horticulture oils:

https://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/

Please keep in mind that any insecticide used to kill a pest insect will kill any insect that it comes in contact with. This includes beneficial insects such as bees. Don't spray when plants/trees are blooming. Read the entire label and use according to directions. 

If you think the problem with your tree is different than what I have described please send a picture so that we can make a diagnosis.