Does it increase in volume when you turn it up?
In my limited experience with tubes, it is always a good start to replace the tubes and see if the problem goes away. A lot of ringing/buzzing/soft intermittent pops/crackles are tube related as the tubes get older (or in the case of minor pops...sometimes when you first put them in).
In my limited experience with tubes, it is always a good start to replace the tubes and see if the problem goes away. A lot of ringing/buzzing/soft intermittent pops/crackles are tube related as the tubes get older (or in the case of minor pops...sometimes when you first put them in).