OK, here's one from the "Live and Learn" file.
Recently, I noticed a very faint crackling noise in the left channel while listening to my beloved '59 Bugle Boys. Crap, I must have a tube going microphonic! The crackling was intermittent, barely noticable and almost...well, rythmic at times. Weird, huhh? I switched tubes, front to back, but the noise was still in the left channel. Double crap! Hmmm.
Tried a different tube pair, same result. Decided I had a bad socket saver. Swapped savers, then removed savers...same noise in left channel. Guys, I have been chasing this faint little crackling noise for THREE days! I have checked cable connections, swapped cables out, nothing solved it.
Finally, earlier tonight I sat back and thought about when this noise first appeared. Well, it was three days ago, just about the time I got my....new Samsung Note2 phone.
I moved phone and charger across the room and the noise is now gone.
I can't explain it, either.
At least you got it fixed!