Tom Fine has already posted a number of good explanations.
There are, however, less-common explanations that you may wish to be
aware of, just in case.
(1) If there is a mismatch between the record machine record and
erase head track position, perturbations in the record bias and/or
erase MAY print to a tape like this. DC-(i.e. permanent magnet) erase
may also cause something like this, but it is usually more of a
"burbling" or what is sometimes called "rocks".
(2) Static electricity and PLAYBACK machine "glitches" CAN print to a
tape without the recorder being in record mode. It's uncommon
(thankfully), but it can happen. Static can be generated by fast
winding in a very dry environment, and depends on cassette materials
including the shell and slip sheets. This is more prevalent with
reels than cassettes.
This clicks can usually be removed (depending on their source) by a
declick/decrackle plug-in for your favourite DAW. The Magix
restoration tools version of this is the best I've yet owned, but I
haven't owned either DC7 nor the high-end Algorithmix version.
At 08:53 AM 2008-09-18, Mike Hirst
>Here's a thing that's been confusing me for some time. I have spent
>the past six months working my way through a large number of
>cassette tapes mostly recorded between 1985 and 1995. every now and
>again I notice light, but significant crackling. This is often more
>noticeable in the right channel, but not exclusively so. On some
>recordings this is louder, on most recordings this is not evident at
>all. This is not restricted to any one brand of cassette, nor is it
>associated with any one playback machine and/or soundcard. Can
>anyone explain this for me?
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.