I never got the WDM driver to work without clicks and pops, but I haven't used it for the past several versions so it has probably been improved. From the Wiki:
Clicks, pops, and static are typically caused by buffer underruns. Because the WDM Driver is re-routing sound in real time, it is more sensitive to buffering settings than normal audio playback. There are two buffers that are important to WDM performance:
The output buffer, found at Options-->Audio-->Device Settings, and
The input buffer, found at Options-->Audio-->Advanced-->Live Playback Latency
If you are experiencing clicks and pops, try different combinations of settings with these two buffers. It is important to note that a larger buffer will not necessarily work better. Different hardware configurations work best with different combinations of settings, so try different combinations large and small until you find settings that resolve your issue.