It Could be a tube going south? Flip flop the tubes ( right and left channel ) and see if the noise travels to the other side..If so you need a new tube set..Probably bad tube...Good luck
Audio Research LS2B Mk 2 has a hissing noise
coming from one one channel (right) when I turn on the unit. There is a delay on the power coming on built into the pre amp for about 40 seconds. After this delay, the noise can be heard and gets quieter and disappears after a couple of minutes. The noise seems not to be related to the volume setting. Any thoughts? Any idea how much ARC gets to look at something? Thank you in advance.