Whistling noise is pretty vague. There are several possibilities. The brakes could be the source of the noise. If they're 85% gone they should be replaced anyhow regardless of whether they are the source of the noise. Otherwise the noise will no longer be your biggest problem. Disc brakes do have what is called a worn pad indicator. This is simply a metal tab that scrapes the brake rotor, making an annoying, scraping noise until the brake pads are replaced.
Other than this, there are several possibilities. Where is the noise originating? Could it be wind noise coming from an ill fitted window seal? Vacuum hoses are another noise source, although they usually only whistle when decelerating or at idle.
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