I am getting an intermittent hum from my 770. I have had it almost two weeks. The hum usually lasts for 15 seconds or so and is loud enough to hear across the room during quiet moments. The hum does not come over the speakers, it is from the amplifier itself. I suspect the source is one of the transformers. Is this normal? Suggestions?

Other than the hum am very pleased with both the 950 and 770.