Unidentifiable popping sound

Popping sounds when there are tubes in the signal chain can be caused when the tube loses contact with the circuit -- bad tube, loose socket, or socket to wire or socket to board solder joint. Usually happens when heating up and cooling down as the bad solder joint  shifts with change in temperature. 

Substitute another preamp to rule this out.

I'm not sure if this will help, but I experienced a similar crackling sound and it was a cable shield that wasn't earthed. Once it reached a certain level it discharged across the inner insulation.

It was a quality brand subwoofer cable with terrific shielding. Problem was it wasn't earthed. After slicing back a slither of the outer protection I gently soldered a wire to the shielding and connected it back to a central earthing point. Fixed.

Thanks so much for the replies everybody. Lots of good thoughts and suggestions for me to experiment with here (and some solid jokes). @wolverine1 I'm really curious to see if this is the same sound you heard with the bridge. Is there any way I could fling this recording I made over to you to see if its a match?

@reubent it's definitely coming from something electronic, it's not from or through the speakers.

I don't know if there's a way for you to send me the recording.  Maybe someone else knows about this.  
 

I do have two suggestions.  If you're using the bridge, try removing it and see if the ticks go away.  Also, try calling PS Audio customer service.  I dealt with TJ a couple of years ago and he was very helpful.  As I mentioned previously, he sent me a replacement bridge to try as well as some beta software.  Unfortunately, none of those things fixed the tick problem so I returned the bridge for a refund.  We never did figure out the cause but I'm very happy with the sound absent the bridge and there are no ticks..

@andbaron 

it's definitely coming from something electronic, it's not from or through the speakers.

I would think that would make it easier to find as you may only need to have one component powered up to hear the sound. So, sometime when you can be in the room, but not listening to music, simply power up only one component for a reasonable period of time to determine if you hear the sound. If not, power it off, wait an appropriate period of time to determine if you hear it after power-off, then repeat with the next component. Worse case, it should take you less than 2-3 hours based on your description in the original post.

Good luck. Hope you solve this soon.