Stringed instruments, if part of the local culture, would provide another hint.
I'd also guess that strings would sometimes be used for crude 'stop/go' or 'come here' communication over distances, via tugs – perhaps ringing a bell or drum at the far end. One person at the 'clapper' element, noticing a hint of vibrations/sound from the distant end, via the string, in more resolution than a discrete tug/ring, might get the necessary insight.
