I think the whistling bit is a throw back from the sailing superstition of ''Whistling up a wind"
Which in the 1800's & earlier would have been a problem with open air venues. It was not untill Inigo Jones used the Roman 'Theatro' Prosceneium arch & roofed over the Phoenix theatre [on the burntout site of the 'COCKPIT' THEATRE London] later on' that FLY LOFTS CAME INTO VOGUE IN pohmy THEATRES . that Fly Lofts came into vogue WHERE WHISTLING COULD BE A PROBLEM. pRIOR TO THAT CUE SIGNALLING WAS POSSIBLY DONE BY USING COLOURED FLAGS ECT.
TIO TIO [HEART 'N SOUL] cHOOKAS [may you always play to a full house]