TinnerFlow™2nd YearDuct Assembly, Seams & JointsStandard Correction
Large flat flange spans may need clips or approved intermediate fastening so the gasket stays compressed across the whole joint, not just at the corners.
On wide rectangular duct, the corners can be tight while the center of the flange bows away from the gasket. That creates a long whistle/leak even with good corner bolts.
Check gasket, corner bolts, clip spacing/detail, and whether the flange face is straight. Add approved flange clips/intermediate fasteners or remake damaged sections so the joint compresses evenly along the full span.
Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.