i don't know exactly how your wind tunnel looks like, but i assumed it's big and that you can't have it on vibration isolators for stability reasons. I think that the only solution to reduce structure borne noise to other rooms is to mount the wind tunnel on a vibration isolated concrete slab. That can be a big job. Depending on the lower frequence you have to attenuate and how much isolation you need, one solution could be to lay the slab on a glass wool matt. But, there's no easy answer and a thorough analyse is required.
I though about it and the vibration source could be just the fan. hence, it migth be necessary to isolate just the fan with a concrete base with spring isolators and a flexible copling to the wind tunnel.
Do you mean vibration that is transmitted through the structure, that is turned into noise elsewhere? Or do you mean noise that is radiated by the structure itself?
It would be very helpful if you could identify the source and frequency of the noise as well.