Ah, well in that case – it’s not us, we’ve got a completely different species. Good to know I’m just paranoid about not disturbing my neighbours…
Yeah, cockatiels are indeed very loud with their whistles. My sympathies. 
Usually, if it’s whistling all day while people are gone, it’s because it’s alone and wants to be sure its ‘flock’ can find its way back – this is why they normally recommend you get two together instead of just one. A second bird could potentially be the (counterintuitive) solution, if these people are not going to get rid of it or leave soon. Best of luck in solving the issue!