Hi!

Wondering what these noises are? Clearly the two chirps when the
phone is hung up is not just "line noise".

I wonder--do you have any cordless phones on the line?

The two chirps you're hearing remind me of what I heard when I listened to
my 900MHz phones on a scanner. Each time one hit the base (e.g. put back in
the base for charging purposes), there would be a brief burst of
communication in the form of tones.

From what I've found out, this is the handset and base agreeing upon a
security code and communicating it to one another. If you're not hanging up
phones, perhaps someone with a similar phone nearby is doing so and your
cordless phone base can hear it. Or perhaps your phones rotate codes
randomly or in the presence of interference. In a heavily enough populated
area, there might not be enough frequencies and security codes available to
give each phone a unique pair.

There's only on reason I can think of that your phone line might have
conversations on it when nobody is using it...and that would be the
possibility that your cordless phone receiver drops into something of a
"listening" mode on any valid frequency...and, again, that there are lots of
cordless phones running on the same band in your area.

Not sure about the 60Hz hum, but it might be a ground loop problem. As for
the ringing type sounds, I remember someone somewhere saying that phone
companies did sometimes put test signals on the line. Do you see these
signals on a regular timetable?

Considering that mixers and phone lines really don't go together, you may be
inducing some of these strange noises when you connect the two.

William