Hi Steve,
Are the units tube or solid state? If they are tubes, then you need to change them (tubes). Tubes, once they have reached their "life expectancy", tend to throw a slight to medium audible "hiss". The solid state is a little more complex. It could be anything from a faulty capacitor, to dirty power. I would recommend changing your power cord(s) to a "higher grade" like the Harmonic Tech Pro AC11 and get a power conditioner. That combo greatly reduces background noise.
Are the units tube or solid state? If they are tubes, then you need to change them (tubes). Tubes, once they have reached their "life expectancy", tend to throw a slight to medium audible "hiss". The solid state is a little more complex. It could be anything from a faulty capacitor, to dirty power. I would recommend changing your power cord(s) to a "higher grade" like the Harmonic Tech Pro AC11 and get a power conditioner. That combo greatly reduces background noise.