I was playing my Blackstar HT40 a short time ago, at very modest level, and out of the blue there was a pop noise, and the 60 cycle hum kicked up quite a bit louder. I shut the amp off for about 15 minutes and turned it back on. This time the hum was lower than usual, then faded to almost nothing for a few seconds, and then it popped again and the hum came back quite loud. The actual amp tone remained unchanged, and the amp otherwise sounds healthy. When I bought this amp it was dead quiet. Then after some time I noticed 60 cycle hum, but there were a few weeks where I didn't play it, and I didn’t entirely trust my memory that it might have been dead quiet initially like I thought, so I just lived with the hum, but this latest event tells me there is indeed something amiss, and that it is even intermittent. When on standby (before and after this latest incident) there is virtually no audible transformer hum. I’ve seen older amps where even in standby there is a good deal of hum to be heard directly from the transformer. This transformer seems to be relatively quiet in and of itself. Any ideas as to what could cause transformer hum that varies?