Gently tap the sides of each tube, in the noisy amp, with the eraser of a pencil. Some tubes have poor internal connections that, "make" after warmup. If you can recreate the noise, or stop it, via the tapping; you've found the culprit.
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?