The determination of the acoustic response of flames as an element in acoustic network in the form of a flame describing function is one of the main elements in the prediction of thermoacoustic oscillations. The possibility to obtain a flame response through the usual methods of dynamic chemiluminescence and pressure measurements starting from an unforced system with incipient self-excitations at discrete frequencies from 20 to 400 Hz in the form of a stabilized flame at atmospheric pressure with a 700 mm tube as a combustor was investigated. Results showed that both the gain and phase can be entirely dominated by the behavior of the self-excitation thus extracting reliable gain and phase information is generally impossible as if the forced and self-excited modes acted linearly and independently. The phase information of the original flame became dominated by the triggered self-excitation although the gain was not significantly affected. Boundary conditions and systems utilized for flame acoustic forcing therefore need to be cautiously controlled when self-excitation occurs.