The water valve on your toilet is a full flow valve.
Ever notice that your water pump doesn't pulse when you flush toilet but it does when using a faucet?
So when flush your toilet there is full water demand on system.
Your friend is hearing that sudden rush thru plumbing system which is vibrating/rattling due to piping not being properly strapped or their water pressure regulator is causing the vibration.
It's not uncommon.
IF you disconnect your hose and quickly open/close that hose bib so there is a sudden demand on system...probably hear the same vibration noise