Vibration when engine hot

My mother in law recently traded in her '99 Suburban for an '03. It's an LT model with the 5.3 V8. She told me that after the engine reaches normal operating temperature that she can feel a vibration in the steering wheel and brake pedal, but only when at a stop. Once she accelarates, the vibration is gone. There is no vibration when the engine is cold. She had the truck scanned but there were no codes. Anyone had this problem or have any ideas? Thanks in advance for the input!