Hello fellow X5er's!
I rarely post but felt I should as my problem is fixed.
I also had the "dry engine" sound on startup. I had taken the X to my favorite mechanic, Uli at Hafkemeyer Motors here in Kansas City for a new water pump (93K miles on the 4.4). 'Didn't mention the startup noise.
Interestingly, when he called me after looking it over, he told me that he noticed I had a leak around the oil filter holder and said the seal was bad. I had him put in a new filter and seal and now, zero noise at startup!
My speculation is that the bad seal allowed the oil to drain from the filter holder when the engine was shut down and caused dry startup noise just like the sound the engine makes on the first start after an oil change.
'Just my two cents. I hope this is helpful to some of you.

Regards,

Scott

p.s. 'Had to change the water pump on the 740 4.4 at nearly the same mileage as the X5. For an otherwise great car, I think the weak point of the bimmers is their cooling system components. IMHO

1999 M Roadster, 22K miles
1998 740i, 120K miles
2002 X5 4.4, 93K miles