i have the Bougle IV, newer version, and a St John MkII - the St John is a much louder clicker.
I think it depends not just on the check mechanism itself but the body weight and especially shape of the reel - or maybe that is obvious. The lightweight series of Hardys, especially the st. Aidan, and I recently realized the st. Andrew, are extremely loud. I think it might have to do a lot with the large, relatively thin face. Maybe it acts a little like a drum, or the shape of a violin. But at any rate while the st. Aidan (and possibly the St. John which is not exactly one of the lwt line, but maybe has a somewhat more similar face and construction) are as loud, if you compare them side by side the Perfects alway have a lot more low frequency components. Both low and high balance are sexy in their own way, but the low stuff in the Perfects is what gets people comparing the sound to the growl of a “big cat”. The St. Aidan when it is screaming sounds a little more like the buzz of large, angry, but extra-scary insect. Maybe like a 2 ft wide cicada.
Another thing I have noticed is there really is a big difference in the sound of reels with nominally similar check mechanisms and weight, but from different companies. Part of it is due to the quality (number of ratchet notches per angle, pawl fit, etc.) and part seems to be a mysterious x-factor. I know some check mechanisms - none of the hardy reels - that I have heard in action are even ugly enough to make one actually want to do without in those cases. I wonder how much of the success of Hardy over competitors in the sound department was accidental, and how much was planned.
Since we live here in a modern world full of machines we probably have a somewhat imprinted notion of what quality, well constructed machinery sounds like without being conscious of it.