My Crack just became one month old since I finished the kit, and I've been enjoying it basically every day. Only recently though I have started using it for prolonged amount of times until I noticed a weird noise appearing only in my left ear. After it happened several times I decided to write down some timestamps. Turning on the Crack at 16:20, the first hours of listening are perfect and when no music is playing my headphones are dead silent. After several hours (around 22) when no music is playing I can start to hear some faint whistling, that really just sounds like the noises I can hear if I tap on my amp's chassis or move it around a bit, so I assume it's coming from the tubes. I thought maybe after hours of use the tubes might somehow become more susceptible to my pc's vibrations?
After a couple more hours though a much louder "whistle" appears seemingly out of nowhere, still only in my left ear, and gets gradually louder until it disappears. All of this in the span of maybe 3 to 5 seconds and it repeats for multiple times. The volume doesn't matter, as the noise doesn't change if I raise or lower the volume. The first time this happened I got really worried and just turned my amp off for the day. Every other time it's the same: if I use my Crack for anywhere between 1 to 4 or 5 hours everything is perfectly fine, but when I leave it on for longer periods the problem appears again.
Today when the louder noise appeared I tried tapping on the chassis and played with the volume knob to see if anything changed, but nothing happened.
After maybe half a minute I tried unplugging my headphones and plugging in another pair, and the noise seemingly disappeared. I thought it might be related to my headphones then, but after I plugged my main ones back into the Crack the loud noise was still nowhere to be found. I've kept using the amp for another couple of hours after that and the only thing I can still hear is the very faint "tube whistle" that I described earlier. I'm thinking that either the noise just stopped on its own while I tested things (at least for the moment), or while switching headphones I somehow moved something related to the problem. Maybe the tubes? I really don't know.
I tried looking for something to compare the loud noise to, to give you an idea of what I'm hearing. One way I could describe it is that it's like a bit lower pitch, but not by much, of the whistle/sound you can hear 2 seconds into this clip (without all of the distorsion around it. the whistle is very "clear" so to say).
Could this maybe be a bad tube issue? This is not a life or death problem, but it does make me really uncomfortable when it happens and as a result I'm not confident using my kit for a prolonged amount of time.
