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I've been trying to figure out this problem since I bought a used e92 328i.
I replaced the tensioner and idler pulley, but was still getting this whirring wump wump noise at idle. The whirring noise was not consistent, it would make the sound probably for 20 seconds, then pause for a few seconds, and come back again. The sound was a deep whirring noise, almost gutteral, I wish I took a video of the before. Idle sits around 580-600 rpm. When driving or giving it gas, the sound goes away and the engine is silky smooth.
Putting my ear to the engine, the sound definitely was coming from the front, and I suspected it had to do with the drive belt and pulleys. The belt tensioner was sagging so that had to be replaced regardless. The noise was still there afterwards but the vibration was a bit less. These symptoms occurred anytime the engine was at idle and at around 600 rpms. On cold starts, no sounds since the engine hovers at 1000 rpm, but once the engine starts to settle, sound comes out.
Anyways I bought a used alternator since some folks (including one in this thread) said that was the problem. Lo and behold, engine now idles with no more whirring noise, and vibrations are 90% gone. It might be related to the alternator pulley, I'm not exactly sure since the used alternator came with its own pulley (not the idler pulley). The idle still sits at 600 rpm but the sound is definitely gone ( I had the engine idling for a good 20 min).
Other things I replaced includes new transmission fluid and tranny mounts (autotragic), new plugs, valve cover gasket, eccentric shaft sensor gasket, valve tonic gasket (all gaskets were leaking anyways), tightened the power steering pulley, fresh power steering fluid. All that was regular maintenance items but had no effect on the whirring noise.
Hope this helps someone chasing similar noises and vibrations.
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