Happened to my father's 335d. They replaced the pulleys, tensioner, and belt. It's an obnoxious noise.
'09 335d Alpine White/Beige Dakota, Sport, Premium, Cold Weather, iDrive, PDC, CA, iPod, Premium Sound
Retired: '93 325i |'99 Jeep Wrangler Sport |'06 330i Sport |'08 335i Sport |'08 328i Sport