I am not sure what does the acronym DNO refer to. Anyway, I've seen one M.Sc. Thesis that studies the noise in three switchrooms. Unfortunately, only the abstract is available in English, the rest is in Finnish

[1]

http://docplayer.fi/3566329-Aalto-yliopisto-sahkotekniikan-korkeakoulu-olli-oksa-palvelinsalien-tyoymparistomelun-selvitys.html

A conference publication, unfortunately also in Finnish, describes the main results of the thesis

[2]

http://www.akustinenseura.fi/wp-content/uploads/2013/08/ap2013_submission_26.pdf

Hopefully, this is of some use to you. To answer your question, the study in [2] finds that the L_A levels are 82, dB 84 dB, and 80 dB, in the three studied switchrooms.