Unidentifiable popping sound

Timing? I have a hit and miss amp.

Do they sound like the weasel?

I had this problem with my PS Audio Directstream dac when using the bridge.  PS Audio was very helpful and sent me special firmware updates and a new bridge to try but the popping continued.  Finally, they took back the bridge.  Things sound very good even without it.

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.