Need GM expert help!!!!!!!

I have a 03 5.3L, the truck runs perfect with one issue. I have what I think is a rough idle that can be felt in the steering wheel and pedal (slight vibration). This only happens at stops, it smooths out alittle when I put it in park but still can be felt as slight vibration I alreay had the TB cleaned and a fuel injection service done, also had the dealer scan for and codes but nothing. The dealer said since theres no codes there nothing they can do "the truck is to specs". Has anyone out there experience this same issue, if so please let me know what was the fix!!!! Also recalibrated PCM. One other thing, my idle is steady and sits just around 500 RPM dosn't jump up or down but I can feel the engine shutter. Please help, Thanks

When I encountered this issue, the dealership replaced the valve seals.