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.
Good Luck...
April 2017
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