Did you put the belt on the same direction it came off in. Did you inspect the angles of alt and idler pulley after replacement. If you didn't line up the notch on idler pulley, you'd have an issue. Also, confirm your alt is mounted right. You can get it on without getting the idler bolt all the way through the back ear...ask me how I know!
Start with confirming the work you did was right...then hunt elsewhere for your sound.
Oh, use a hose to confirm location of sounds...as a stethoscope...and did you replace the vent tube to the alt? If it was off, it might sound a bit louder than before.
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Mangofy your E46!