In my other post (Is it idiomatic to say "He doesn't drink much water at school"?) I quoted an expression.

He doesn't drink much water at school.

And asked whether the expression sounds natural.

And then I realized that I've used sound(s) natural a lot.

I searched a bit on ELL "sound natural" got 857 hits, "is natural" got 279 hits. Which seems to indicate that the former is more commonly used than the latter.

Are both expressions idiomatic?