Rod knock. What you are hearing is the beginnings of a rod bearing failure. The clearance between the bearing and crank shaft has opened to the point that when oil drains in to the crank case after sitting there is too much gap for that initial start up period. During that startup period there is a second or two where very little oil is present at the bearings as it has drained out. It takes the engine a second or two after starting to build oil pressure and distribute that oil through the engine. This is not heard on an engine in good shape as the bearing clearances are tight enough to prevent the knocking.

I'm not sure if the X5 will do this, but most drive by wire throttle cars can be cranked without starting if the gas pedal is held down to the floor. This is a good test to see if what you are experiencing is a bearing issue. On cold start on cars that are capable of this, one can crank the car for 5 seconds to build oil pressure then start as normal. If the knocking sound is not there after cranking for 5 seconds then it is definitely bearing related.