I live in an apt and when others run water it squeals loud but even if no one is running water I can still hear noise

Could be any number of things, bad pressure regulator, loose faucet washer or a leaking flapper and noisy fill valve. The fact that you hear the water running without use would tend to lead to a toilet issue. However, what kind of heat do you have? If hot water, you could be hearing a recirculating pump.