Was the taste present when you first kegged the beer? It is possible to pick up infections and off flavours from your tap (sorry, faucet) and/or beer line.
Normally I thoroughly clean the tap and lines after each keg by running a warm PBW solution through (followed by a warm rinse, then Starsan). This works fine for my forward sealing Perlick taps, however I recently noticed diacetyl developing which wasn't present when I kegged. This particular line was connected to a Celli tap, and sure enough when I pulled it apart it was full of gunk. After a good clean of the tap and line, the diacetyl flavour disappeared (thankfully). I now ALWAYS disconnect the beer line from the keg if I know I will not be using it for some time. I will also be completely pulling down my taps after each keg is finished.