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? ????
Try another pair of interconnects from preout to amp in. If yo're getting hum when you jiggle the able it suggests that something is wrong with the cable or the RCA jacks at one of the ends. Do you get hum on both channels or just one?
I did try brand new interconnects. Humming is still there. It really is more of an overall hum that seems to come on slow but at other times it is there when amp is powered up.