As of May 2021, an experiment conducted by Learn Metrics has found out that the quietest dehumidifiers in the market are those who produce sound softer than 60 dB. This number h is the standard of regular dehumidifiers. They have found that the quietest is the GE APER50LZ at 47-51 dB and the TOSOT GDN50BA-A3EBA2E at 51 dB.  So, if you don't want to soundproof a dehumidifier, buy a new one instead.