If you can access the wall cavity, adding rock wool (mineral wool) insulation will make a big impact by reducing vibrations and sound transfer. 

@Leland S.This is going to be very costly and the issue it is not resolving is vibratory noiseHaving built many building there is sound and vibration transmission - you can dampen the sound (again costly) but if it’s hardwood floors or a kid above it’s not going to stop the pounding / jumping .Is this a real issue brought by the tenant?

Just remember there is to types of noise - sound and vibration and each one is treated very differently.

If the fan is not properly attached to framing or a for-the-purpose brace, they certainly can vibrate and become loose enough to fall.Having the ability to move air when temps are uncomfortable provides pleasant relief, and is FAR less costly to operate than an AC, which may be an important issue for the Tenant.

So, besides now protecting the floor if this tenant wants to keep using it because these free standing bags can move around, they also are noisy and can cause vibration.

This will decrease sound transfer and vibrations.

I am in 1 unit and rent out the other unit.And My occupied unit I rent out to the roommates. and one of roommates make so much complaint about 2 month.She just move in 12/2023 and left another 9 month of the lease.She refuse to pay the rent because lack of maintanance, there's vibration sometimes behind the shower, cause lack of the support.I already hire 3 plumber and they say it is okay to use it.She never belive it and still want us to repairShould I repair it?

Remember there are two types of sound - acoustic (noise) and vibration.

The issue is that a train track runs behind it and the vibration is horrible (literally feels like an earthquake day and night).

Carpeted floors above would also helpBut you have sound and vibration and vibration is impossible to get rid of.