I am looking for a single word to describe this case.
Lets say someone makes a claim and tries to back it up but fails at doing so. These words do sound pleasurable perhaps filled with platitudes, often used to communicate good intentions but lacking meaning or reason. It's the best I can do to describe this.
The word is close to something like this "those are just a bunch of syllables"
It is a mistake to assume that every concept imaginable can be expressed by a unique word. English is lexically rich, but not that rich. Sometimes you must resort to more than one word, a phrase, a clause, a sentence, or even a book.
There are nouns that mean close to what you seem to want to convey. "Nonsense" is a common noun to express lack of logical or factual substance, but "nonsense" by itself does not imply that it is pleasant or plausible. A "commonplace" and a "platitude" mean that a thought is both commonly held and expressed in a formulaic way, but that does not make the thought nonsense. "Vacuous" is an adjective that means lacking logical and factual substance, but it does not imply "pleasant" or "plausible." This is where a thesaurus comes in handy.
Of course, once you let yourself combine nouns and adjectives, you have a wealth of possibilities: "sweet nothings," "meaningless pap," "vacuous platitudes," and "platitudinous nonsense" are examples. And you can concatenate adjectives: "vacuous but plausible platitudes" expresses the sense you seem to want with a euphonious consonance.
EDIT: User Nate has suggested that "specious" is an adjective to consider.
Your question seems to seek a noun. I have three suggestions:
Flimflam =
1 : deceptive nonsense
2 : DECEPTION, FRAUD
Or you might use
Waffle = to talk or write a lot without giving any useful information or any clear answers:
I also like
Drivel = something written or said that is completely worthless; nonsense
SPECIOUS is my “go to” for that.
... apparently good or right though lacking real merit; superficially pleasing or plausible.
Let's review that again:
Yup, it's perfectly correct.
OP, you're asking a "how do I write this sentence in my short story" question. There's no SWR. Here's a few random common-ish phrases that might fit.
empty chatter
rambling
content-free
devoid of substance
lacking content
All of which you can google for exemplars.
Single words that may work include
specious
vacuous
florid (notice sense 2 in the OED, versus your "those are just a bunch of syllables")
But the key point is that it is a mistake to assume that every concept imaginable can be expressed by a unique word.
Further, it's a mistake to confuse the act of writing or speaking an excellent description of a specific quality in the actual specific situation or events (fictional or factual) of which you are writing or speaking, with, finding "a word" that fits that very specific quality; to put it more briefly, there comes a point where one has to write, not just thesaurusize.
When, say, Richard Bach writes, say, The river was wine beneath our wings to begin a story, there is no SWR ("The river was profound ..."? "The river was dramatic ..."? "The river was spiritual ...?" "The river was intoxicating ...?"); he had to Actually Write.
Mitch has offered, in a comment, two very apt terms for this concept:
bullshit
and
word salad.
They, in particular the former, obviously cannot be used in very formal contexts, but the OP did not specify the required level of formality, so there is no reason not to post them as answers. They probably capture the intended concept more accurately than the other terms offered on this page; in fact, some of the other terms offered seem to be essentially euphemisms for bullshit.
There's a taxonomy to be had of sentences (or discourses) that are syntactically correct and stylistically consistent and acceptable, and superficially truth-like (see 'truthiness'), but do not match correspond to true statements. Some labels already exist for these properties, eg untruth for something that is not true/does not describe things that are the case, white lie for a statement that is only slightly wrong, enough to not be questioned in order to bypass an ugly truth, or a whole range of words about nonsense (bunkum, balderdash, gobbledygook (which are themselves look like somewhat random collections of syllables)).
'bullshit' is talk that is neither true nor false but is purports to be factual, and in distinction with 'lie', the utterer isn't trying to mislead with a lie but instead mislead with irrelevant information. (sometimes 'bullshit' is actually a euphemism for 'lie')
'word salad' is less competent than bullshit - it's a string of words that sound like they go together (by collocation frequency) but end up barely or not even making a coherent point. Here the implication is that the utterer themselves doesn't have competence in the subject area rather than any kind of intentional misdirection about the subject.
Sophistry = the use of fallacious arguments, especially with the intention of deceiving
M-W gives:
: subtly deceptive reasoning or argumentation
Try Tinsel. Per Merriam-Webster, the word means:
Something superficially attractive or glamorous but of little real worth
disfigured by no gaudy tinsel of rhetoric or declamation
—Thomas Jefferson
Technobabble describes arguments that use the language (jargon) of science without actually being in any way scientific.
