OṂ, that syllable truly is Brahman.
That syllable indeed is the Supreme. …
Knowing that syllable, indeed whatever one desires, is his.
(Katha Upaniṣad 2.16)
The vast majority of those mantras are nonsense syllables such as Hring, Hung, Hum, Phat, sparks from the blazing furnace of aboriginal superstition whence the system arose, or from the equally superstitious stores laid up in the Atharvaveda.
(Farquhar, An Outline of the Religious Literature of India)