Sound energy is propagated in three ways-
Radiation, which is the direct sound from the source,
Modulation, which is the resonance of materials and objects that are responding to the energy and
Conduction, which is mainly structure-borne resonances.
The easy way to find what's vibrating is to move through the room, listening for where you think the sound is emanating. The reason I wrote "...where you think..." is due to the fact that the sound may seem to be coming from one place, when it's actually coming from someplace else.
Remember to check heating ducts and vents- these can be made to vibrate very easily and it may not sound like they're the source of the sound.