なにやら means "something or other", "somehow": the speaker notices something but can't say exactly what it is. It goes in front of the part it is vague about – a noun, a verb, an adjective. It is also written 何やら.
。
I can hear some kind of strange sound.
。
There's some kind of nice smell.
。
He seems busy with something or other.
It can also stand where a noun would, as "something".
。
He's writing something or other.
なにやら or 何か
is the everyday "something" and says nothing more. なにやら adds that the thing is unclear, and often that it feels a little odd or uneasy. It is more at home in writing and storytelling than in conversation.
。
I have a bad feeling about something.
Lingaku