Possibly a failing capacitor that eventually "forms" after voltage is applied for awhile. Might be able to locate it probing with an analog ohmmeter (preamp powered off). While you're under the hood, watch those high voltage power supply electrolytics; they can hold a charge for a long time if no bleedoff is provided.
Is my preamp dying a slow painful death?
My British Fidelity A3 (solid state) preamp is about 12 years old. It now needs about five minutes to warm up before making music. At powerup, there is a noticeable buzz from the speakers that cycles around five seconds of noise, five seconds of silence, which after a minute or two turns into five seconds of music (if a source is playing) and then five seconds of silence. Everything is fine after about five minutes. What is happening? Is this the beginning of the end?