My Fender DR, probably 10-15 year-old model, has recently developed a peculiar, pronounced hum. It happens only occasionally, following a cold startup, gradually (1-2 minutes), increasing loudness, only up to a certain level. Its steady pitch is about the frequency of an open D string. It is not acoustic feedback, it is not affected by the position or proximity of the guitar. It appears to be an oscillation that coincides with the time the amp is warming up. Switching the amp off, then on again fixes the problem (yes, just like a computer). Could this be some power tube issue?