It sounds like your heating/cooling system in the apartment is using chilled water (and hot in the winter, have fun in the in-between seasons!). That conditioned water runs through a heat exchanger, then the thermostat controls the fan to move it into the apartment. The pumps that move the water may be the source of the noise, or it could be the fan. Some, but not all, use a valve to shut the water on/off into the heat exchanger and that could make noise if it leaks a little. It won't hurt to insulate the pipes with some foam sleeves, but there may not be much else you can do about it. If the pumps are getting old and noisey, lots of luck geting them to replace them unti they actually fail. The pipes probably feed all of the units, so travel through the apartment somehow. The noise can travel - any place the pipe is in direct contact and not isolated from the structure, it could be acting like the speaker coil and vibrating the walls/celing, etc.
I use a hydro-air heat exchanger for my heat, but I used rubber isolators to keep both the pipes and the heat exchanger isloated from the ductwork and the fan housing...it doesn't make any humming or vibrational noises.