My money says it's either part #8 or #6 in the drawing StephenVA posted (or both). Those pipes are plastic and the ends toward the front of the engine will rot out and leak. It typically only happens in cold weather, but will eventually get worse. I've replaced many of them for this exact reason.

If you know where to look, you can see coolant pooling around the junction of #8 and the side of the cylinder head. You need a bright flashlight and look between the oil filter cap and the side of the head. But it's not easy to see. And, unfortunately, it's not an easy fix. The intake manifold has to be removed. Of course, once that's done, actually replacing the pipes is a 10 minute job.

If I ever have to remove the intake from an M52TÜ or M54, I always replace those pipes - even if they look okay. They're relatively cheap and I'd hate to have to come back later since it's so easy to do with everything taken apart.

Replace BOTH pipes, even if only one is leaking. It's only a matter of time before the other goes. And the labor is the same whether you're replacing one or both.

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