Check to be sure the end of the STS rod is not butted against your stabilizer. Also - you can put a piece of foam in between the two, this seems to knock down some noise also.
With my FireCat - it was quietest with the bumper just a hair away from the string at rest, however with the Nemesis, AX and AZ - they are the quietest with the
string right up against the bumper at rest.
Actually on my X's and Z I have the bumper turned so the slot is 90 degrees to the string rather than the string hitting in the slot.
One way to find out if the STS is the issue is to just take it off and shoot to see if that eliminates the noise.
Another thing may be - have you shot other bows with the STS? They do produce a thunk when you shoot.
And have you had someone else shoot it while you stood 5ys or so away and listened?
good luck on yours!
2010 nemesis camo, 2011 nemesis carbon fiber,
2010 onza 3 camo,