To add to this, sometime after I overhauled the accessory drive, I started hearing some whistling again. Then i started noticing a very very slight miss on idle.

I decided to check the intake system. Intake elbow was cracked badly at the creases and had small tears. Disa valve flap was also broken but thankfully had not come apart yet.

Changed both out and all quiet again.

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04 X5 3.0i auto
03 X5 4.6is