I would jail them for that !

More like a pipe with corrosion perhaps some condensation happens with the changes in temperature and the air or liquid inside makes a movement hence the noise. Just a wild guess

Could be a Syphon for heating too maybe a little clogged

Hopefully it is not that ....I mean my friend did contact the previous tenants but they never mentioned the cat....then a neighbor saw the cat from a window and called the fire department!.

Have you tried calling the owner of the apartment? Something similar happened to my friend and 3 weeks later they discovered that the tenants abandoned their pet cat.