I had a similar issue with my 2017 3661RL. However, my issue was sawdust. I hoisted my shop vac up on the roof pulled both shrouds on the units, carefully removed the styrofoam separators and vacuumed everything out. Moved to the interior removed all duct grills on both supply and returns and fished shop vac hose thoroughly back and forth. That got the job done. I repeated the process about a year later when I was fixing collapsed duct work. Not sure if there are any other sources of styrofoam other than the separtors in the units themselves. I think your on the right track to start there first.
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Gordon & Victoria Hallof
Louisville, Ohio
2020 Ram 3500 HO Cummins Aisin 6 Spd
2017 3661RL
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