Popping noises


I noticed today a very slight 'popping' sound in my left channel. Noticed this noise while my tube amp was idling and I am presuming it is a driver tube in the amp section...is this a fair guess or could it be something else?
128x128daveyf
Does this happen only during warm-up; perhaps within the first half hour only?

B
Seems to happen during warm up only. After I took the amp off idle and played various pieces it seemed ok.