Free will is as philosophical a question, in that sense, as there is. Kantian ethics should not represent itself as having a solution to it. If the problem of freedom is a philosophical open wound, then the right way to think about Kant's utterly unacceptable theory of noumenal freedom is that it is the salt that philosophers have a professional obligation to rub in the wound so that they can't forget about it.

Kantian Ethics (2008) - Ch. 7. Freedom