While certainly some fake it, as personal opinion I am convinced that some are sincere and are not consciously faking.
There is, however, such as thing as being sincerely mistaken.
Glossolalia (to use the technical term) also appears in shamanism, paganism, and voodoo; it has been demonstrated that it can be rapidly learned by most people; and it has been demonstrated that where Pentecostal preachers have influenced churches not previously exposed to it who then adopt the practice, their vocalization patterns are related to his with high frequency.
(Yes, there has been serious linguistic study of this, always finding it to not have the full characteristics of language.)
All these things indicate that the mind can enter a state where nonsense syllables can be generated in manners sounding just like what is in question here.
Lastly, those who speak in tongues almost or perhaps actually invariably use only phonemes already known to them, whereas other actual languages invariably have additional or differently made sounds. It is unreasonable to imagine, if granting angelic languages, that angelic language has American Southern pronunciation.
Anyone who is trying to speak or indeed is speaking another actual language adapts his pronunciation, or his accent if you will, towards that of the other language. The overall sound changes markedly.
This doesn’t happen with tongues.
So, if granting that the mind can enter a state that generates nonsense syllables, but a person is taught that this is a spiritual phenomenon, I think it’s fair to state that it could easily happen sincerely.
I have no doubt that some close associates I’ve had in the past, as well as for example the pastor I previously mentioned, were sincere and not faking. Generating it themselves, yes, but not knowingly aware that it didn’t require a spiritual gift or knowingly aware that it was perhaps much sound, it was signifying nothing, in any language.
And thus, since in the KJV from which they obtain the expression “tongue” means language, they were speaking in no tongue. But I am positive they were not aware of that.