Try cleaning the tube pins. Make sure the bottom of the tube (the glass) is clean as well. Use your cleaner and a Q-tip to scrub the tube pins.
Using contact cleaner in tube sockets
My ARC VT100mkii which I just got, has a crackling from one of the channels. The tubes are new. When I first installed the tubes, it was fine. The following morning, I powered it on, and the crackling started. I swapped tubes with the other side and it was fine for a while. This evening, it started again. Will using contact cleaner damage the sockets? It is quick drying cleaner.