The property is old, and part of the floor is lower, it is bellowing. Someone suggested leveling it with hardboard - can you advise/any tried and tested suggestions ?
Search on Youtube - uneven floorboards - there are several videos.
Laying thin hardboard might solve your problem but it's also likely to create other problems - over time foot traffic may well cause the fixings to work loose creating a bouncy and squeaking floor.
If the property is rented then it's a job for the landlord - if it's your own place and you intend to stay there for some time then have it done properly.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix general d.i.y.