Probably the same sound as mine (and a lot of other GS's) has. It only occurs when it is warmed up and when you come to a stop. The sound is pretty loud and the first time it happens you think the engine is about the explode :) After a few seconds it dies away.
Mine stopped doing it when I used SHell Semi-synthetic oil, but now I use Castrol GP 20W50 (mineral oil) and the sound is back.
As long as it dies away after a few seconds it's ok. Mine does it for 2 years now and it still hasn't exploded yet :mrgreen:
Have you changed the oil already? (Do you know what brand and viscosity of oil was used in the past? / how often changed)
Does it sounds lik clacking? I have this sound coming from my GS as well. Somebody told me it's just my timing chain.
It's disconcerting, but a normal GS sound, especially since the bike has some miles on it.