Might be a dead/dying valve - I had a similar problem with my Copland CTA-305 (making a slushing sound in one channel, starting and stopping) - when all valves were changed all was well and haven't had an issue since.
Have you already eliminated the amp or any other possible source? Are you using any line (a.c.) filtration? I've noticed various degrees of very low level hiss with the different line filters I've used. Other than that, it may be a tube dying, as has been stated.
Since it is coming from just one channel I would first suspect a faulty tube. Did you try swapping tubes one at a time between the left and right channel to see if the hiss follows a tube swap? Also, are you sure it's coming from the preamp and not from the amp, cable or speaker?
Since it is coming from one channel I would first suspect a faulty tube. Did you try swapping tubes one at a time between the left and right channel to see if the hiss follows a tube swap?
Hola...usually if the problem is volume dependent, then it is coming from the pre-amp. Also, a change in the harmonic texture, will be a bad tube too. Good idea is as Craig is saying...swap one and listen, then swap another until the problem changes to the other side...that is the culprit...happy listen and happy new year!
Roberto.