Musical Fidelity strange noise. HELP

My relatively new Musical Fidelity KW550 integrated amp recently started to have static-like noise only on left channel. The strange part is that it doesn't change by volume control or mute. It's exactly the same volume regardless the input or level of the volume control. It seems like once the unit starts to warm up. The noise gradually decreases and disappears.
Anybody can help me with this please? Thanks in advance