This is a spectogram of a dial-up modem handshake sound.  Via FlowingData. When I plugged in my first modem for the first time I heard this sound sequence and thought something was broken. Took me a few goes to figure out it was supposed to sound like this. Finally got connected and discovered bulletin boards. #waybackwhen

And Oona Raisanen has shared this image explaining what all those sounds are.

Dialup Modem Visualised