Air carrier Captain reported return to departure airport due to vibrations and unusual loud noises on climbout.

Narrative

Flight ABC was a routine flight with an original destination of ZZZZ. We departed a few minutes behind as a passenger asked to be removed due to health reasons and a bag pull was required. Once in the air we departed and took off on our planned route. About thirty or so minutes after takeoff we were climbing when my FO noticed an unfamiliar noise that then became very loud. I then noticed this noise as it grew louder and louder and we began to experience a vibration in the flight deck. The vibration began to get worse with my FO's side of the flight deck worse than mine. Both of our rudder pedals were vibrating and our yokes were vibrating very aggressively. We began to check to make sure we had no recall messages and everything was pressurizing normally. We had no recall messages and all the door panel lights appeared to be closed as we suspected a door was not closed at first. I stopped our climb at flight level 290 to work on this problem and did not want to climb higher in case something else was going on we were unaware of.ZZZZ Center had us on their frequency when I asked my FO to take the flight controls and communications while I called dispatch on the crew phone to explain the situation. After speaking with our dispatcher we decided it was in the best interest of safety to return to ZZZ as there is no procedure to address this vibration and loud noise we were hearing. My FO and I agreed to go back to ZZZ as we both had our concerns about continuing to a terrain environment in Location A and our flight over water with this unknown issue we were having. I called the flight attendants to ask if they could feel any unusual vibrations in the forward or rear of the aircraft and they also said they heard the loud noise up front and small vibrations felt. I advised them of the situation and that we would be returning to ZZZ as a result and gave them their briefing due to the overweight landing we had to do. ZZZ Center took us back from ZZZZ Center as we were being vectored back throughout this interaction. Once with ZZZ we advised the situation and that a hold was necessary to try to burn some of our fuel off and complete some more items before heading back for the approach. Once in the hold we calculated a significant amount of time to burn almost 11;000 pounds of fuel which would take too long and with this noise and vibration getting worse we [requested priority handling] due to the overweight landing we needed to do to get on the ground and not delay anymore time.We were vectored back to ZZZ where we landed on Runway XX uneventfully and had safety vehicles follow us after we landed to the gate where maintenance was there to assist.The cause of this problem we experienced was due to a ground service panel being left open which according to Maintenance personnel it could be seen flapping in the air as we were taxing to the gate. This door which was open and loose could have broken off had we climbed higher and continued the flight to ZZZZ which could have left this for entering the number two engine and causing an even larger problem especially with the terrain destination we were headed too. Maintenance completed the write ups for both the vibrations and noise which was a result of that ground service panel left open and completed the other writeup for the overweight landing inspection needed due to landing approximately 10;700 pounds over. The ground service panel was the problem as per maintenance which poses a safety risk for any flight. Ground personnel should be sure this panel is secured prior to salute given before we taxi away for takeoff.