The answer is generally no, there is nothing wrong with your NU Audio Card.  This sort of clicking noise is normal among all high-end audio cards with operational amplifiers and should be expected.

However, let’s first determine that this is what you are hearing.  First, confirm that the clicking noise is occurring near the NU Audio Card, as opposed to hearing a clicking noise through your headphones or speakers.  If you are hearing the latter, then this is a different issue and is likely related to your audio playback or some other audio issue. 

If the noise is coming from the NU Audio Card, then what you’re hearing is the line-out OP-Amp switching on or off. This is completely normal. You will normally hear this when your system turns on, when the system resumes from sleep, if the driver resets for some reason, or you enable/disable the Speaker Playback function within the NU Audio Software.

If you typically use only headphones and wish to disable the click, go into the NU Audio Software, and deselect the Speaker Icon in the Playback Menu. It may click at that point, but this will disable the click from occurring unless you revert your settings to default or reinstall the NU Audio Software.

After clicking on the Playback button, you should hear a click, and the button will become grayed out.  When greyed out, you will not hear any audio out of the line-out connectors, but you can hear audio out of the headphone port, so long as the headphone icon is still lit up, as shown below.  When you have successfully disabled the line-out port, it should look like this:

Keep in mind that disabling the line-out playback neither improves nor harms the audio playback for headphones; similarly, using this method to turn off the click does not extend the lifespan of the card. This is simply another option for preference.