its is probably the vibrations like you suspect
the pictus catfish has lateral line sensors which detect vibrations, and I know that they can make that croaking noise, so maybe the vibrations somehow pique their interest.
This might not be relevent but i was first told by my lfs that i should keep all high pitch and loud noises away from my tank. So i sold my LOUD stereo system. Then i thought how can i hoover the carpet. But when i hoover the room the fish don't even seem to notice. Weird.
Ps i am male so no sexist comments! Lol