Could be the pressure cap venting too. Mine had a leak at the smaller upper "reservoir tank" connection. The small hose connection was intact. However, the inner part that holds the o ring was broken. It broke due to vibration over time. It was a perfectly broke off plastic ring too. It took time to find it. I actually found it by replacing the upper hose.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)