Noumenon and "mind only"

Yes, Noumenon is our Tathagata nature, our Buddha Nature, Tao, God, Allah, Brahmin... whatever that thing is that is beyond the dualism of existence and non-existence: the Mind.

Noumenon is the opposite of phenomena, things that are perceivable through the senses or in Buddhist terms are a part of the five skandhas (see Surangama Sutra).

This is why it is so important to become more detached from phenomena because they detract the Buddhist from realizing the only worthy goal of the Noumenon, the Source of all things, the Us that is beyond time and space.

The more one develops a worldly way, the more one is attached to meaning-ful phenomena be it money, sensual pleasure, relationships, and the infinite dualities that trap us to the wheel.

The more one grows in the spiritual path, the more one is able to perceive the foundational unity of all things. The profound truth in Emptiness. One is unable to be so attached to any particular thing--not with a numbness, but with a crystal clear clarity developed through the understanding of Dharma and most importantly through the transformation of consciousness itself that takes place by success in meditation/awareness/vipassana.

This is the Right Path as espoused by the Eightfold Path and the last question you asked is different question, that would take an entire paper to discuss, but basically yes.

Well actually in the teachings with regard to the sense-consciousnesses, six are enumerated:

1. Auditory consciousness (sense of hearing, the ear, its "objects" like sounds)
2. Visual consciousness (sense of seeing, the eye, its "objects" like sights)
3. Scent consciousness (sense of smelling, the nose, " " smells/scents/odors/aromas)
4. Taste consciousness (sense of tasting, the tongue, " " tastes)
5. Tactile consciousness (sense of touching, the body/skin, " " textures, etc.)
6. Mind consciousness, or mental consciousness

The raw perceptions of the first 5 themselves have no dialogue, no commentary, no "story" ... the sixth one can meld and blend the first 5

So in Buddha's teachings, there is nothing outside the Six sense-consciousnesses, and therefore it seems untenable that one could argue that mind is "knowable but beyond the senses" unless one is referring to a different breakdown of the sense faculties, their focal points/objects, and the combined consciousness.

You should know that it is possible for a yogin to "withdraw" from the five coarse sense perceptions and experience the luminosity and delight of the sixth sense-consciousness. There are instructions in the suttas/sutras, most notable is perhaps the Buddha's discourse on breathing meditation, which is in most traditions the foundation to introspection, insight, attainment, and liberation.

I think that this might be the result of a translation problem. The concept of a noumenon in western philosophy is very foreign to Buddhism I think. In western philosophy a noumenon is the opposite of a phenomenon. A phenomenon is something that is experienced, whereas a noumenon is something that is somehow outside of experience all together but somehow exists anyways.

Most forms of Buddhism don't settle well with the idea of any kind of reality having some sort of existence that is independent like that because it falls into the error of essentialism, or at least it appears to.

I think you may be coming across this word in an off translation or something relying on such a translation, because I suspect a translator may be trying to translate the concept of 體用 (tǐ yòng, usually now translated as essence and function) as noumena-phenomena, but I don't think this is a very good comparison at all. Definately the term 體 does not mean noumena, but something more like the nature of something.