The whistling sound could be coming from some weird combination of acoustics within the muffler as it rots from within and begins to change shape internally. But more likely, what you’re hearing is the sound of a dying turbocharger bearing. Generally, these should be almost silent, but as they wear, they get louder and louder. The turbocharger sits inside the exhaust system, which is why you can hear it through the tailpipe.
Left to its own devices the bearing will eventually collapse, taking the turbocharger impeller/turbine with it. The bigger danger is that some of the small fragments of metal from the bearing will be free to enter the intake side of the turbocharger where they will be travel through the engine causing all kinds of destruction. That’s possibly what will happen if you ignore it.
