Chris M and Aaron came out due to issue with fluttering sounds. Performed diagnostic. Turned furnace on. Heard fluttering sound coming from return grill. Went down to crawl space where unit was. Found filter door not locked in. Found no filter was present. Placed new 16x25x1 filter into filter slot. Closed filter door and locked tightly. As soon as this was performed fluttering sound had stopped. This is warranty on previous technician. No charge. No warranty due to no actual repair made.