I just bought a triplex (we have tenants upstairs and in the basement, we live on the main floor and also have a basement) the construction is not that old, it was built in 1986. Recently, I have noticed 2 noises coming from the pipes (i assume its the pipes).

1- There is a distinct whistling sound that lasts for a few minutes when we use our shower or flush our toilet on the main floor. The sound goes away when we flush our downstairs toilet or close the downstairs toilet valve. My father in law seems to think we would have to change the valve in the downstairs toilet. The tenant downstairs also hears a sharp whistling sound when she flushes her toilet that lasts a few minutes. The upstairs tenant doesn't have this problem

2- Recently, Ive been hearing a sort of waterfall noise from time to time, like water escaping the pipes. I have no idea what that could be.

We're probably going to call a plumber but I just wanted to know what it could be and if it's a serious problem.