Lampizator DAC Amber crackling noise

As per title, mi Lampizator Dac Amber makes some sort of crackling noise. The noise, I think, could be due to a bad tube or tubes, However, what puzzles me is that the noise is in both channels, not really loud, although audible from my listening spot and it is not preset at all time, I mean is not continuous in nature. I have to said the noise is not the same in both channels, i.e. the same noise does not comes out from the speakers at the same time . Am I right supposing the noise is due to a bad tube o tubes? Since the noise is in both channels could it be due to a bad rectifier tube (6X5)? What could be the best approach to solve this problem? Any help/comments would be very much appreciated!  

Thanks in advance,