A hearing aid system and a method of estimating ambient noise in a listening device includes an input transducer and an output transducer, an electrical forward path between the input transducer and the output transducer providing a forward gain, an electrical feedback path comprising an adaptive filter for estimating the acoustic feedback gain from the output transducer to the input transducer. A method determines the quality of a critical gain measurement for a listening device. The method comprises a) monitoring the energy of the first-difference of the filter coefficients of the adaptive filter over time and b) applying a predefined threshold criterion to the change in energy content from one time instance to another to determine an acceptable impact of the ambient noise. This technique may e.g. be used for the fitting of hearing instruments where background noise is variable.