It is probably the taste of Taurine (http://en.wikipedia.org/wiki/Taurine).

Taurine was originally extracted from bull's balls and hence when you drink a RedBull, you are giving a bull a ... lets say, doing a bull a favor :-)

(Taurine is not extracted from bull semen nowadays, of course ( probably coz this is not scalable :-p ) and it is claimed that the whole Tauine-Bull thing is a myth... but myths are fun to propagate)

http://www.google.com/search?q=taurine+semen