Hiss

[paulears]

IF this noise is really OTT, rather than a background hiss, compared to the wanted signal. it could be worth checking for unusual voltages on the audio lines - especially if the individual components seem ok on their own. It's a while back, but I had a Soundtracs mixer probably fifteen years ago that had a fault on it, there was DC superimposed on it's output from a fault in the PSU, and the Behringer crossover it was connected to objected by generating large amounts of hiss. I spent ages at the Behringer end, to only discover the real fault by luck when using a meter to check the cables, where I thought the issue was hiding.