This sounds like a very low level noise that does not interfere with the desired signal. All electronics will have some level of noise, tube based circuits tend to have a little more. With careful circuit design and layout, it can be minimized, but never completely eliminated. If this was loud enough to interfere with your signal, or even just audible from the listening position, it might be a solvable problem. This is not that, it's just the level of noise that the preamp makes.