All species of toadfish and midshipman do this. The exact sound varies with species and are most prevalent during breeding. It can range from a grunt, to a hoot, to humming. It is produced by "strumming", which is when a specific muscle is vibrated against the air bladder.
I have seen them in fresh, brackish, or full salt. I kept mine in full salt as that is what it was acclimated to when I bought it. Very cool species. I have read that some toadfishes can make up to 15 different "sounds".