Pure sounds is a layman's analogy. What they actually mean is a single
frequency radio transmission (like you get in a quiet patch from an AM,
or FM broadcast station and is still distinctly present but mixed with
other frequencies when there is sound on the transmission).
Most SETI search systems are designed specifically to look for
transmissions with this characteristic, so nearly all candidate hits
will be of this form (although at least one professional contributor to
the list believes this is the wrong thing to look for). Most signals
detected that meet this condition can be traced to local sources (even
the PC used for the analysis) or earth satellites. Other such candidates
have never been verified by other teams.
Examples of a couple of unresolved, and rather more false hits are on
I repeat that we are assuming that hits will in this form and designing
equipment to find them on that basis, rather than seeing such signal and
then assuming they are hits.
If you have a communications receiver, but it into SSB mode and tune it
through a broadcast band, the whistles you hear are the "pure tones" that
earth is leaking (although broadcast transmissions are not optimised for
their ability to leak).