Coralrose:
I realize it must be difficult for anyone to inspect you claim as the noise problems seem to be occurring late at night. Do you have anyway of recording or videotaping the noise in your apartment.
I have steam heat and usually there is little clanking but overall it is pretty quiet. However one night at about 3am it sounded like one of my neighbors was hitting the heating pole pipe in the bathroom. The banging was so loud I could not even tell exactly where it was coming from...above, below or the apartment next to me. We have had one or two similar incidents which sounded more like water banging thru the pipes and definitely not loud like the first incident. Is either close to what you are hearing?
I sleep with an air purifier running (white noise drowns out alot) and it softens the sounds from the uncarpeted clods above me.
Besides taping the noise, do you have any neighbors who could corraborate the problem? strength in numbers you know...