That cross over pipe is a heavy casting. Can't imagine it leaking. It has gaskets on each end (the two heads) and then the orings for the metal tubes in the middle.

The heater hoses run behind the block. Probably worth taking the engine acoustic cover off and looking around with a mirror to localize the source better.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles