Hi Guys, I have had the popping problem and it is totally different to a clunk.
I have a 4.4 V8 X5 2004 model and I noticed the popping noise while I was doing an engine oil change . (routine stuff right) How this noise started was when I noticed the Power Steering reservoir was leaking from the bottom connecting tubes/hoses. I looked in the reservoir and noticed the fluid level was low and then when I turned the steering wheel (with Engine on) it created this popping noise. It is due to air entering the power steering system. It then gets pumped through the whole steering rack and can cause this noise on the way back out.
Solution, replace or tighten the hose clamp assemblies feeding the steering rack and pump.(see if this is leaking as often the factory clamps come loose replace clamps with new ones if required) then drain and refill the entire system with new power steering fluid. Once its full you will notice the noise never comes back.

The clunks on the other hand can be due to many steering related under-car components (and suspension components) mentioned in this post and elsewhere on this site. I would suggest putting it up on a hoist and observing whats happening while the wheels/ steering is being moved about. If you're not sure then take it to a good BMW specialist.

I hope this helps some of you.

regards