Citation C-501 pilot reported loud noise coming from the main cabin door area and a rise in cabin pressure altitude in cruise. Pilot descended; continued to destination airport and landed.

Narrative

Upon reaching cruise altitude of FL380 and leveling off; a loud noise occurred coming from the main cabin door area. The Master Caution went off and the cabin altitude light on the control panel illuminated. The pressure gauge indicated the cabin pressure altitude was rising rapidly and the differential pressure was falling. The transponder code was immediately reset to XXXX. Immediate procedures were initiated and an immediate descent was established. ZZZ Center was notified of the loss of pressurization. Upon reaching 12;000 feet MSL; the plane was leveled in cruise configuration and the pressurization started to return to normal. It was decided to remain at 12;000 feet MSL for the remainder of the flight and land at the original destination. A new transponder code was given and intentions were transmitted to ZZZ Center. Upon landing at our destination; I called a number for ZZZ Center and informed them of the situation of no injuries and no damage. The aircraft was inspected by an A&P Mechanic who discovered the door seal had developed a leak and would need to be replaced.