BMW e46 330ci Can any help me diagnose this noise? 

[TKFTW]

It's probably the alternator vent rubbing the belt it also could be a bad pulley

[AMMA-wezz]

It sounds like a belt noise, however, that’s not what you want to change alone. On BMWs and e46 in particular, change the belt tensioner, idler pulleys, water pump, thermostat, and the belt. Do them all and you should be good. All parts would be less than $150 total. You can narrow down the problem by removing the belt and spin each pulley separately including the alternator pulley ( if the bearing is bad, change the alternator), however, you still would need to replace all the parts I mentioned. To make your life easy, you remove the fan clutch, the cooling fan, and then all the parts I mentioned. There are plenty of videos on YouTube to do this job. Good luck.

Note: If there are oil residue of the belt, you will need to change the oil filter housing gasket before changing all the parts above and that requires a little skill as you have to remove the alternator and PS pump plus everything else mentioned above.

[Axelforever]

You need to go to the mechanic and ask him like a sort of oil to put on it if he says that there are no oils to put on there then you need to change the strap maybe

[housespider]

never owned a car that's made as strange of noises as an E46