ruby said:

Peak Warming Man said:

buffy said:

Several recordings at Graham Chapman’s site, link

Yeah that’s them, I don’t know where I got the hissing sound from.

I had a pair of tawnies that raised a number of broods in my old gum tree over a few years, so I have definitely heard the hissing as well as other odd noises from them in addition to the lovely low oooom oooming.

This article says so too!
https://theconversation.com/hard-to-spot-but-worth-looking-out-for-8-surprising-tawny-frogmouth-facts-146484
Tawny frogmouths are quite vocal at night and have a range of calls from deep grunting to soft “wooing”. When threatened, they make a loud hissing sound.

Next time I’m chatting with the sister, I’ll ask if she’s heard her tawnies hissing.