Anything like that (and other means) is still classified.
Since I have absolutely no knowledge or any contact the Op Sec on my posts is probably trivial. Just guessing, I suppose, would be how to categorize, FWIW.
Seems to me, though, acoustic “camoflogue” would be good. Get a tape of “ normal whale bowel sounds” and emit that...
Even after hiliary?
Wasn’t there some reference to that type of thing in a Tom Clancy movie?
You can find these things out, as long as your source doesn't intend to tell you.