Are there words starting with the letter "y" pronounced as a vowel sound and can be proceded with "an"?
Middle English allows us "yghe" (eye) and "yren" (iron). The first can easily be found as "an yghe" in old books. The second is mostly used as an adjective (e.g. "an yren smith"), however, "an yren bounde cofre" (an iron bound coffer) can be found too, which I believe designates "yren" as a noun. The OP simply asked for can be proceded with "an" so the designation as a noun isn't strictly a requirement, but I felt strangely compelled to try to meet that criterion.
However there are two animals that start with "y". The "yllatron", another name for the "agouara" (crab-eating raccoon!) and the "ynambu", another name for the "tinamou" (a South American bird, similar to a partridge).
So there are geniune modern English usages of "an yllatron" and "an ynambu".
Hence the answer to the question is "Yes".