Full Idea
Leibnizian monads clearly satisfy Kant's definition of noumena.
Gist of Idea
Leibnizian monads qualify as Kantian noumena
Source
Sebastian Gardner (Kant and the Critique of Pure Reason [1999], 06 'Noumena')
Book Reference
Gardner,Sebastian: 'Kant and the Critique of Pure Reason' [Routledge 1999], p.201
A Reaction
This needs qualifying, because Leibniz clearly specifies the main attributes of monads, where Kant is adamant that we can saying virtually nothing about noumena.
Related Idea
Idea 5568 We cannot know things in themselves, but are confined to appearances [Kant]