The clicking sound is probably the valve in the intake manifold (the one that adjust the manifold length... the proper German name escapes me now): it's common.

Replacing it on the X3 is tight, but can be done. The one thing you'll have to be careful is when you remove it not to have the shaft falling in the manifold. The shaft can come loose and when pulling it out might fall down. Move slowly and be patient.

The running lean is a vacuum leak: very common too. Two things to check:
1) the branch in the manifold going to the idle control valve cracks. Replace it
2) the crankcase ventilation systems leaks: in this case there's not really a definitive way to diagnose it. You'd have to replace the valve/separator and the plumbing around it.
I normally start with the manifold, and if the problem comes back replace the crankcase ventilation system.

Dirty MAF, bad O2 sensor or even failed cat are normally bad diagnoses and very very rare causes of the symptoms you have...

I hope this helps! Let us know...