I have a question related to a furnace filter in my daughter’s condo in Chicago. The furnace is an 80,000 BTUH unit installed in an upflow position. It is the only unit in a ~1200 sq ft single-floor condo. The cold air return is on the right side with the opening into the furnace cabinet located on the lower right. When changing the removable filter, I noticed a filter-like material covering the opening with it located interior to the cold air return duct. Please see attached photo. At the bottom of this photo, the channel for the removable filter can be seen. This material was not removable and has collected dust, etc. but is difficult to clean. I have never seen this so am curious why it is there? I am thinking that the return duct is internally insulated (for sound suppression) and this material is the internal insulation that was never removed from the opening when installed? I can’t help but think it is restricting airflow, especially since it has collected dust. Can it be cut away? Thanks for any comments.