is it just a subjektive feeling or have you compared the sound w/ an other engine
Muffler is ok? Or can it be a intake noise? Or chances the speed of the engine and let it roar - goes the engine out of round? Hard to say what it can be if you can't hear the sound... maybe you can specify
Welcome to GTTalk. You need to find out what the recommended idle speed is for the engine. It may be in the owners manual. Then you need to get a tachometer and adjust it to the correct speed. It may be set to idle quite fast in order to keep the cooling air flowing at a good rate through the engine cooling fins.