maddog,
I had the same issue after I bought some NOS tubes. I thought I would clean the connections with DEOXIT to bring back the shine and theoretically improve the connection. Silly me, I forgot to dry off one of the tube's pins from the wet DEOXIT. It was then throwing some aggravating crackling and drove me nuts until I took my air compressor to the socket and gave it a good spritz of air. That took care of my issue. It was only in one channel though. Good luck!!