In this paper, we address the problem of monaural source separation of a mixed signal containing speech and piano components. We use Discrete Energy Separation Algorithm (DESA) to estimate frequency-modulating (FM) signal energy. We design a time-varying filter in the time-frequency domain for rejecting the interfering signal. An estimation of the FM signal energy employs instantaneous signal properties that are localized both in time and frequency. We present experimental results which demonstrate the advantages of the proposed method using real audio signals.