I haven't read all of the posts here as I don't have time but as I recently fixed my coolant leak in my N62 and did my own coolant transfer pipe replacement (The ORO pipe).

Turns out it was my radiator. I replaced pipes, the coolant transfer and all o-rings, the expansion tank as it was really starting to shit me, it kept going so I did a real thorough inspection of every inch of the system and found a very small rust spot on a metal section at the top of the radiator. The problem is because it is a pressure system you fix one spot it just pops the next. The radiator was always the main culprit but once I had everything else buttoned up it gave me the signs that I could see.

Anyway radiator replaced and it all was fixed... hope that helps

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2004 E53 4.4L N62