"constant undercurrent" is a correct and usable expression in written English.
You could use it to describe a situation that has a pervasive, recurring theme or influence. For example, "There is a constant undercurrent of uncertainty and fear in the air."
There's a constant undercurrent: What's next?
Perhaps because this has been a constant undercurrent since early last year, it often goes unremarked upon.
They are emblems of a new poignancy forming a constant undercurrent beneath Mr. Uys's often bawdy humor.
Where other pavilions were shrouded in solemn quiet, this one had a constant undercurrent of soft laughter, whispered conversations.
"There was a constant undercurrent of a kind of financial and professional desperation," her friend Bernie Weinraub, who was then the Times' Hollywood correspondent, says.
There is a constant undercurrent of chatter from the Pentagon that this July 2011 date isn't really, but Obama aides insisted to me that it is.
Although her message was somewhat profound, there was a constant negative undercurrent which read: What is wrong with our educational system?
As a concept, it's pretty basic, though the nightmare's enlivened by a constant, thrumming undercurrent of high school homoeroticism, which manifests itself in lingering crotch-shots and constant close-ups of half-naked teens.
Mark E Smith and The Fall are one of the constant, cryptic undercurrents of English music.
"High seriousness with an undercurrent of irony is a constant feature of Brazilian art, literature, cinema and even music".
From the pompous shiny-faced politicians in the George Square "palace" of marble, to the occasional obscene example of contemporary wealth, through to the sectarian undercurrents of religious and football divides, it is a city in constant flux.
