Try having someone else shift while you listen under the car to see how far back the noise is coming from. There is a crossmember underneath that you can use as a point of reference. As you can see here and below, the crossmember is roughly where the middle universal joint is (the bolts coming down from the body on the left and right are where the crossmember bolts on). There is also a similar joint at the very rear of the driveshaft.
White 2007 Alpina B7
Supercharger clutch/bearings (check DIY), sunroof seal, quarterto8's 6000K angel eyes, bumper side mounts
Black 2002 BMW E66 / 745Li (sold)
KW v3's, blue angel eyes, 8000K headlights & HID fogs, rebuilt alternator, sunroof seal, CCVs, NGK Laser Platinum plugs, sway bar bushings, battery, timing & valve cover gaskets, valve stem seals, guibo joint, mechatronic sealing sleeve, trans fluid, alternator bracket mounting gasket, sanded headlights...