After installing a new DC system I noticed a huge improvement in DC noise (unit is now essentially outside) but now notice the noise created by my 2 air filtration units. The worst offender is an older one made by Total Shop. It's not much more than a metal enclosure with filters on both ends and a single speed motor driving a squirrel cage fan. Are there quieter units made? As an alternative to buying something new is there a way to quiet the existing unit?