Air carrier crew reported rumbling noise and vibration during climb out. Crew elected to return to departure airport where door seal was observed to be sticking out of door.

Narrative

ZZZ ZZZ1; Ferry Flight. Climbing through FL200; we heard a loud rumbling noise accompanied by vibration in cockpit floor (lasted approximately 15 seconds). We checked all of the systems pages - everything was normal. Through FL280; the exact same noise/vibration for the same time; though FL280; the same condition; only lasted approximately 5 seconds. We opted to stay at a lower than filed altitude; and slow the speed back. We sent ACARS to Maintenance Control; asking if they had any history (we did not see anything in the log history). We then had Dispatch 'Call Me;' which they did on a frequency and patched Maintenance Control on as well. Maintenance saw nothing in their computer; nor did they see a history. We confirmed all parameters were normal in system pages. Maintenance Control felt it was safe to continue; the First Officer and I felt comfortable with that. We agreed; and informed Dispatch if we had another occurrence; we would elect to divert to a suitable airport. We entered a Maintenance write up electronically. When we went back to use the lav; we immediately heard loud airflow... appeared to be from Door 1L. Again; pressurization was normal. Shortly thereafter; we received a message from dispatch that Maintenance Control would like us to return to ZZZ. We confirmed with Dispatch that we had the appropriate fuel. Weather was VFR; and ZZZ came back to open the Tower. We landed; and when Maintenance opened Door 1L; he immediately told us what was wrong. He showed us the seal around Door 1L was loose at the top; and showed us a screenshot of the door (from before he opened the door) with the seal sticking out of the top of the door.