EMB-145 First Officer reported the cabin altitude was unusual during the climb. A loud noise was heard and while preparing for an air turnback; one of the FMS failed along with a pack overheat.

Narrative

We took the runway out of ZZZ on Day 0. We departed Runway XXR. The plane had three MELs in the flight deck - autopilot INOP; thrust reverser INOP; and gust lock INOP. We took off out of ZZZ. On climb-out; I went through my climb flow and found that cabin altitude was at 800 ft. As this seemed unusual as it should still be lower; I opened the QRH to the pressurization chart. At which point; I found that the delta p was close but the altitude in the cabin was closer to a much higher altitude - that of about 18;000 ft. in the chart. Coming through 10;000 ft. we got a buffet in the controls and heard a loud noise. We called the FA (Flight Attendant) and asked if they heard anything abnormal in the cabin; which she responded yes to. They said they thought it was the wings and we asked her to go look. At this point; ATC was passing off to another controller. We asked to stay on frequency; get a vector; and a level-off; at which we did at 15;000 ft. and advised ATC we needed roughly 5 minutes to troubleshoot and would get back to them. The FA then called back up saying there was a whistling noise coming from the main cabin door and service door; but the wings were fine. We then told ATC we would need to return to ZZZ. From there we asked the FA to take their jumpseat and fasten their seatbelt in case the door fully depressurized. Also; conducted a briefing and advised her about 25 minutes till on the ground. At the same time; we were overweight so opened the QRH to the overweight landing checklist and ran through that. We then had an FMS 1 Fail and a Pack 2 Overheat message. We pushed the Pack 2 Button out as the aircraft can pressurize on one pack till FL250. I sent Dispatch a message telling them we were returning to ZZZ for a possible overweight landing. We requested lower with ATC at this point to burn more fuel; and ended up down to 5;000 ft. During this; Person A was flying and I began to set up an approach back into ZZZ. We then requested [Runway] XXR to come back in and did the normal pre-landing checklists. We then came into land. We missed disengaging the yaw damper before landing as we typically disengage the autopilot and yaw damper at the same time and were task saturated. By the time we landed we were 300 lb. roughly under maximum landing weight and came into land. During landing the rudders were tightened up but had movement where we could align the plane to the runway and make a safe landing. Upon reaching the gate I noticed the yaw damper still on and we included it in our maintenance write-ups once the aircraft was back safely at the gate. Person B was the FA; unknown employee number. While we have a standard call of yaw damper; autopilot disengaged; landing" in our books; due to task saturation and the autopilot being disengaged; we missed it. I believe we could add this into the landing checklist as a secondary level of verification."