**Update**
Oh, you're talking about Stairway. I believe the noise you're hearing is the microphone they used to mic that guitar cabinet. It's not overdriven type distortion because the frequency is much too high to be the guitar. More like the microphone diaphragm physically being moved too much by the SPL.
You might find this interesting, it's a dissection of Stairway and a discussion of the recording techniques used for it's production.
http://www.led-zeppelin.org/reference/index.php?m=assorted7
Taken from that site : The fanfare at 5:35 marks the end of Mr. Nice Guy. Page has three guitars spread across the field -- twelve-strings on the outside and a six in the middle. Notice the distortion here? The clicky noises that happen with each strum? I really doubt that was intended. That sounds to me as the the preamps in the mixing board overloaded, but I find it hard to believe Page or Andy Johns wouldn't have noticed that while they were tracking. It's possible that the distortion happened somewhere further down the line, or it's even possible that Page intended it (although I really doubt it). I can tell you that the same distortion exists on the vinyl, so it's not a CD error.
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Yeah definitely check your source file. There is so much 'do it yourself' these days in the music industry that sometimes mistakes get through.
Anybody listened to Foster the People? There is some distortion on a couple of tracks on that album.
If it's on the guitar part, then more than likely it was their amplifier. Guitarists rarely direct box into a recording sesison. They use their amplifier, and put a mic in front of it to record the output. If there is a crackle from the amp, the crackle gets recorded and makes it all the way to your ears.
Now if you ever suspect it's not the track, as Obo suggested, frequency sweep to try and find obvious problems. A big problem I have had on DT880s is hair getting into the driver capsule and rattling on some bass notes. Beyerdynamic put a 1/4" (2mm) piece of very thick felt in between the driver and the foam ear piece, yet my short stiff little hairs some how manage to work themselves all the way through. I clean them monthly now to prevent the need for more open heart surgery.
The other common cause of distortion or crackling from a driver are dimples and dents.
Headphones are constructed with as thin and lightweight a driver material as possible. Usually it's some kind of plastic and since thin plastic is pretty floppy, they will mold curves and folds into it in order to make it stiffer and support itself. It looks nothing like a speaker cone anymore, it's full of rings, dimples, curves, etc. Anyway sometimes for various reasons one of these curves might get a wrinkle in it and that wrinkle will cause a crackling sound on certain frequencies. Depending on the construction of your headphone you may or may not be able to disassemble and expose the driver surface to pull those dimples out with a little masking tape. Sometimes the solution is as simple as blowing very gently on the driver.
Edited by Kodhifi - 2/16/13 at 11:26pm
