CRJ-900 flight crew reported during the climb a flight attendant advised them of a whistling noise near the passenger door. Passing FL250 the flight crew received a passenger door red warning and advised ATC of their situation; and decided to land at the nearest airport. The flight crew landed safely at the alternate airport.

Narrative

I was the pilot flying for the flight to ZZZ. After departing ZZZ1 on the ZZZZZ; around 10;000 ft. to 11;000 feet; the Flight Attendant's call came in; and the Captain handed over the radio to me to manage while he responded to the call. Initially; I assumed the call might be about cabin temperature; so I instinctively reached to pull up the page. However; the Captain soon informed me that the forward Flight Attendant reported a whistling noise near the passenger door; (the Captain (CA) initially suggested wind as a possible cause). After a quick check; we found the pressurization to be normal and continued our climb without further calls or concerns raised from the forward attendant.Upon passing FL250; we were alerted with a Passenger Door Warning and an aural door warning. I immediately began to level the aircraft and informed ATC about our situation; while the Captain made a brief command PA to the Flight Attendants (FAs) and passengers to be seated". I called for the QRC for the door message; but the Captain; upon checking; found no relevant information. He designated me as Pilot Flying (PF) and started to run the QRH. At this point; we began to continue to descend to 10;000 feet. ATC cleared us initial to 13;000 ft. thereafter to 10000 ft. The Captain informed me that the QRH called for us to land at the nearest suitable airport and I noticed that ZZZ2 wasn't too far ahead.ZZZ2 was also suggested by ATC as we continued our descent and advised them we would likely be landing at a nearby airport. As we continued to descend; I noticed an increase in cabin altitude; and shortly after; the Cabin Altitude warning appeared. We both completed the memory items; the Captain then took over as Pilot Flying; and I followed through with the QRH procedures. After this was completed; I called to check on the condition of the cabin crew and passengers. The forward Flight Attendant (FA) relayed that everyone appeared to be in good condition and I shared this with the Captain.At this point we were at a safe altitude (no oxygen masks still being worn); and in the ZZZ2 airspace; we were in communication with approach; receiving vectors for a delayed approach into ZZZ2. I had sent a request for performance data at ZZZ2 and realized we would have an overweight landing due to excessive tanker fuel.The Captain came up with a strategy to reduce our fuel weight for a safer landing to prevent landing overweight. We requested and received permission from ATC to exceed 250KTS to burn more fuel; but never went faster than the original 290KTS we were at when the door message displayed.We increased our speed initially; then elected to lower the flaps and gear to create more drag.As the Captain continued to check the weight; re-check the QRH and the test items; I maintained positive control of the aircraft and received delay vectors and communicated with ATC. Once our fuel levels and aircraft weight reached a safe landing weight; we informed ATC and prepared for our approach into ZZZ2. The Captain asked if I was comfortable completing the approach and landing. Being familiar with the ZZZ2 area; the airport; and this specific approach I said yes. We requested and executed the RNAV XX into ZZZ2 without issue.We taxied to the terminal without issue. After we parked at the gate; we completed the shutdown checklist. After this; the Captain received a call from the chief pilot to discuss the incident; while I stepped out to coordinate with the Ground Crew about passenger arrangements. As we waited for further instructions and engine maintenance to assess the situation; I learned that there would be space available in the terminal for our passengers.We began the process of deplaning. Before initiating this; I confirmed with the Flight Attendants that everyone was fit to disembark and that the Ground Crew had set up a safe ramp for passenger exit (a normal sloped roll-up ramp was used; not the normal aircraft stairs or jet bridge). Once assured; we facilitated the passengers' movement into the terminal.Simultaneously; I liaised with the fire and engine maintenance teams; providing them with the requested details about our aircraft registration and the flight number.After all the passengers had disembarked; the Captain continued his conversation with the chief pilot and maintenance; handling the administrative and reporting aspects of the incident. My focus remained on ensuring a smooth and safe transition for our passengers from the aircraft to the terminal and coordinating with emergency services to manage the situation effectively. I was informed that there were no passenger injuries and everyone was inside the terminal."