Cayin 300B integrated amp-popping sound

array of reasons:

tube(s) going bad -- swap tubes between channels one by one and see if problem moves to another channel.

cap(s) going bad -- visually inspect circuit for any out of shape caps(mostly DC): leaking, bubbling, bending.

poor cold solder joints -- visually inspect under loupe solder joints mainly around ground path, but would not hurt to inspect the entire unit, but start from RCA input plugs and speaker terminals first especially if you've been recently reconnecting components and wires frequently. Cold joints could look like ones cracked, rash-shaped, dull.

As you go over your unit, clean the dust inside the chassis.

A lot of 'popping/static' sounds are caussed by poor pin contact. To begin your search I would first eliminate this possibility by cleaning the pins. If it continues thereafter, I would switch tubes from channel to channel and see if the sounds change channels as well. In fact I think I would do this first as often just the removal and replacement of tubes is enough to clean the pins sufficiently for good contact.

If after switching tubes (and thereby eliminating the possibility of a bad tube) and cleaning pins you are still having noise problems in the same channel it could well be poor contact in the tube sockets themselves. You can correct this by simple re-tensioning of the contacts in the sockets.

Best to call Steve at Cayinusa. Since he is the US distributor, he is closer to the issues then anyone else. Steve is always helpful.