Hi, Putt putt boats draw in water which flashes into steam and is then ejected mostly as fluid. Given that the water was delivered to Rossi's device in pulses, it seems possible that it also ejected water in pulses, at least to some extent, as the leading edge of each pulse flashed into steam. Since the outlet was directed down a drain this might not be noticed.
IOW "that's the pump" may also have been the concurrent sound of a pulse of water being ejected. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
