Could be a room mode - use this formula to work out your modes
f = V / 2d
f = Fundamental frequency of the standing wave
V = Velocity of sound (330m/sec (1130 ft/sec)
d = Room dimension being considered in feet (length, width, or height)
Other standing waves occur at harmonics of the fundamental frequency - that is 2, 3, and 4 times the fundamental.
cheers
John