I ended up replacing the pump (I have ASR so it was the larger, ASR/ABS combination hydraulic pump) and it cured the rattling/ratta,tatta, type noise.

To bleed the ASR/ABS pump you have the car running, open the bleed screw to let fluid out, then close the bleed screw. The pump should run for approximately 30 seconds - don't turn the car off until you hear the pump stop running. Also - make sure you have adequate brake fluid in the reservoir at all times.

Also - replacing the pump isn't hard at all. The most difficult part is loosening the brake lines. Make sure you have a brake line wrench also known as a flare-nut wrench (11 or 12 mm I think).

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE