What is year and model of your X5. Please add it to your signature. If the rattling wasn't there before you replaced the timing cover first step is to check your work. There are no coincidences when working on vehicles.

If the proper amount (or more) of oil is in the engine it will show on the dipstick. Oil should be checked when the engine is warm and off. When the engine is off the oil pump is not functioning. Did you overfill the engine?

Why did you replace the upper timing cover?

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