B737-800 Captain reported a high pitched squeal on the flight deck from an unknown source resulted in a precautionary return to departure airport.

Narrative

On Day 0 while operating Aircraft X myself and First Officer had a mechanical event that required us to return to our departure airport. During initial climb out passing 8000 feet; we were presented with a very high pitch squeal on the flight deck that we had never heard before. I had transferred the PF duties to my First Officer so I could pay closer attention to where this sound could be coming from as we continued our climb. We received no abnormal indication; light; or abnormal feel on the controls but as we were climbing the sound seemed to be getting stronger. My First Officer and I were communicating what the issue could possibly be and he brought up that it could probably be a seal that has malfunctioned and recommended that we stop our climb. I agreed with his theory because the sound did get louder as were climbing so we stopped our climb at approx 14;000 feet to assess things further. I monitored the pressurization and it was on schedule and did not look abnormal. After talking it over with my First Officer; we agreed that with the loud squeal; a potential pressure issue; we did not want to risk going to altitude. We made the decision to return to field and I decided to not request priority handling because; besides the noise; all other system operation was normal and we were light so we did not have to come in overweight. I watched the pressurization closely and it descended on schedule and the sound subsided at 3.3 psi and completely went away at 3.0 PSI. I remained PM for the rest of the flight as I handled my management duties and briefed the FAs that we would be returning to ZZZ for a normal landing. Rest of the flight was uneventful.