Ever been on a really long airline flight, say across the Pacific? After a while, even the noise level of an airliner can get tiresome. For the ISS crew, the noise is 24/7. There's no escaping it. All of those fans, pumps, and other pieces of equipment may individually make only a little noise but together, it adds up. Even relatively moderate noise levels can cause hearing loss over time. This apparently was the case with almost all of the Mir cosmonauts. Even with the lower noise levels on the ISS, the constant drone interferes with communications and sleep. Miscommunication and fatique are the sources for a lot of mistakes and potentially could lead to accidents.

According to this old BBC article, the noise levels on the ISS are as high as 74 dB, especially in the Russian Zarya module. From this article:

Captain Jonathan Clark, MC, USN, of the NASA Johnson Space Center, Houston, TX gave an interesting presentation concerning noise issues in human space flight. NASA is concerned about the acute effect of sound on crew performance on the International Space Station (ISS) and is looking at strategies to evaluate and reduce acute, chronic, and delayed effects of sound. Crewmembers have complained that noise levels on the ISS have caused impaired speech intelligibility of non-native language (Russian Cosmonauts also work on the ISS.) and often complain of sore throat from talking loudly in noisy environments. On the space shuttle, acoustic dosimetry revealed maximum noise levels to be around 70 dB (A). No hearing loss occurred, but the crew reported significant effects on performance and communication. TTS and PTS have not been observed in the U.S. shuttle program. TTS has been reported for 100% of Russian Cosmonauts and PTS has been identified in 27 of 33 Cosmonauts (82%). Long-term space flight has produced 30-50 dB HL of NIHL in the 4K-6K Hz range. On the International Space Station, there is constant background noise in the 56-60 dB (A) range. There is concern that the Astronauts and Cosmonauts are not able to obtain "auditory rest." NASA recommends hearing protection when noise exceeds 60 dB 24 hour Leq. The use of insert earplugs and passive or active noise reduction headsets are recommended.

TTS = temporary threshold shift & PTS = permanent threshold shift,