I was just plotting the freq. response of my WM40 and at exactly 50Hz, there is a strange oscillation, no clear frequency can be heard, just sort of a wobbling, varrying frequency, and the dB meter goes crazy wobbling between 70 and 80 dB. Why is it doing this? Is it the room acoustics, could it just be the resonant frequency of the enclosure causing it, or something else? It does it in 2 different places in the room. It is polyfil stuffed also.