Science des rêves, La
You don't know what it is, you have no idea. It's like being deaf and singing freestyle to the tune whistled to you on the phone. And every time you muck up (and you muck up a lot), person at the other end of the line gets mad at you, and you can't even hear that. But you feel that you're out of synch, you just know you must be, and you're embarrassed and sweaty. You keep on singing, too scared to stop, to be alone with the dead receiver, painfully unsure if there is someone whistling.
And when it's over, it's just, "Ha, what was it?"