Ditto on what Kerryf said.
Any time a car sits for awhile, gunk can mess up the fuel pump and filter.
There is also a fuel pressure regulator on the fuel rail that regulates fuel into the injectors. That could be fouled.
You should also consider disconnecting the fuel line to the fuel rail, putting a bucket under it and cranking the ignition to pump some gas and see what the flow looks like.
The fuel filter is located under the car in the transmission well by the front seat, I think. As for the pump, located near the filter for this model, I think, you should hear it when you are at idle, but it should not be terribly loud. If it is really loud, it's probably toast and can't pump higher volume.
Hope that helps.
Current cars: '99 E39 wagon (200k+), '99 E36 Cabriolet (220k+), GMC Yukon Denali (186k+, but who cares?)
Prior BMWs: '99 E39 (120k), '94 E34 (220k), '93 E34 Touring (320k), '86 E30 (275k), 76 E12 (175k), '74 E10/model 115 (2002 tii) (110k)
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