Whistling with the AC engaged on the W210 is a common symptom of low refrigerant. I'd first check that before I started digging in elsewhere.

The facelift 210 allows you to run some HVAC diagnostics (including system pressure) from the climate control itself. Still, if you're considering adding refrigerant, I'd use a gauge set to first determine the pressures and, of course, to add some if needed.

Good luck.