If you are driving normally, following speed limits (kinda), not towing then that would be low mileage for 3.0. Do you know if it got better mileage before you noticed the noise?

I am very hesitant to suggest nothing is wrong but so far there is not enough information to say there is. As previously suggested, I always use an auto stethoscope, a must have tool for a DIYer, to trace a noise to the source. Can purchase one at most auto part stores. Just move the probe around the engine until the sound is the loudest. Check close to the injectors and close to each cylinder. Also check the water pump. Checking at the water pump should give you an indication if the noise is coming from a pulley or tensioner but getting a close to to them without getting into the belts is better. If it is coming from the bottom of the engine you will likely hear it with a probe on the oil pan but it is better to each cylinder on the block from there. Sometimes you can narrow it down if a cylinder is involved by probing the base of the valve covers.

IMO, with something like this don't ever buy parts until by troubleshooting you feel sure you have found the issue.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas