Possible solutions for a 2 foot crawl space under my floor system?

Hey guys,

First time poster. I'm dealing with a low end resonance/boom/floor vibration coming from the 2 foot crawl space (550mm to be more specific) beneath the hardwood floors.

I've already renovated and signed the lease (so finding alternative location is no longer an option (lesson learned)

Looking for the most cost effective way to mitigate this issue.

We are on the ground floor. (21 sqm room)

Have considered the following options.

- filling the space with crushed gravel (bricks or sand) then finishing with a 300mm concrete slab.

- insulating under the floor boards with knauf earthwool

- adding MLV ontop of the floor boards and maybe even a layer of floor leveler to increase mass

- would adding a layer of chipboard, underlay then thick carpet do the trick?

- building a new floating floor system on top of current floor

- float the original flooring system with better isolation from the joists

feeling abit overwhelmed and lost in terms of how to proceed. ideally looking for the most cost effective solution but also want to hear the best solution (if cost wasn't an issue, assuming this might be pulling up the floor and filling the whole space with a concrete slab?)

Any help/suggestions is much appreciated!