For grid-following inverters, an accurate grid impedance estimation (GIE) enables many beneficial applications. Conventional active GIE methods, such as the PQ variation method, intentionally introduce power disturbances around the system's steady-state operating point. However, it might bring undesired reactive power into the grid and its accuracy can be influenced by phase-lock loop (PLL) phase angle variations. To address these challenges, this letter proposes a noninvasive GIE method, which utilizes PLL's inherent operating characteristics. It only requires a single steady-state operating point transition that naturally occurs during the system's normal operation, eliminating the need to inject deliberate PQ disturbances. In addition, by considering PLL phase angle variations, this method also ensures accurate estimation results across diverse conditions. Experimental verifications are provided, which demonstrate the effectiveness and accuracy of the proposed method.