As a consultant that specializes in acoustics and sound reinforcement/AV design, I have to ask, what exactly are you trying to accomplish? Depending on what you're trying to do, involves different remediation strategies.

Are you trying to control the amount of noise that is transmitted *outside* of the server room? For example, are there adjoining offices to the server room that are getting objectionable noise from the equipment in the server room?

Or, are you trying to make life a bit more bearable *inside* the server room? For example, are there some poor souls who have to sit at a desk inside this room and do their work?

Throwing a bunch of treatment on the wall may be of some help, but keep in mind if much of the noise is in the low end, you are likely dealing with vibration being induced into the building's structure, which implies isolation solutions as well. Wall treatments that are effective for low frequency noise are generally several inches thick, and can be rather spendy. Also don't forget that such treatment must be fire rated in order to comply with fire regulations.