Ho Hum

I am running tube preamp,tube phono stage,tube headphone amp into the amp section of HK 730 integrated.I have been running it this way for 6 months no problems. All of a sudden there is a wicked hum when they are powered up. After some tests its seems as if the headphone amp does not experience the hum at all when listening to the phones. When it hums the preamp is very sensitive to the touch and makes severe thumping through the speakers. Then the problem just disappears for a listening session mysteriously. The next morning it is happening again. Any thoughts? Tubes? Ground Loop? ????