As I live in Bangkok, I often stay here. For the past few months they have been building a new part of the hotel (including Spa). This unfortunately is next to the Regency Club and Presidential Suite area.
During my last stay in December, I noted that the building work is at a point where it is quite noisy and the building work is constant - 24 by 7 (under flood lights at night). The building site was only about 5 meters from the door of my Regency Club room.
I would recommend to stay in the normal part of the hotel which is the other side and far from the building work. If you pay the right rate, I am sure they will still give you access to the Regency Club lounge.
It will be a beautiful spot when the building work is finished. The new part will copy the Residential Spa Concept that Grand Hyatt Bangkok recently finished.