Eruvin 104: Somebody's Knocking (Should You Let Him in?)

Scatter salt on the ramp in the Beit HaMikdash, to make sure that the priests don't slip. Also, several places to draw water on Shabbat. There's a beraita that suggests this was a Temple-only list, but the Gemara says that you can put straw down on a muddy road that's hard to pass. Anywhere that is, not just the Mikdash. How is the straw case different from the salt case, that the latter is Temple only and the former is everywhere? Also - the story of Ulla visiting on Shabbat, when someone comes and knocks on the door - which Ulla finds to be a desecration of Shabbat. But isn't this a case of infinite regress - so many things make a sound - from games to incidental everything. Perhaps knocking on a door to be heard is different, because the aim IS to make a sound. Alternatively, as long as it's not to create music... you're probably good. Plus - on drawing water on Shabbat - in the Temple. Is that because doing so makes a sound?? But are these wells specific wells or kinds of wells? And either way, the greater concern seems to be watering one's garden. Except not in Mehoza, where there was no chance of using the water wheel there to water one's garden... Until they started using the water...