Date: 201711
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuselage Skin
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Passenger; Person Flight Crew
When detected: In-flight
Result: General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: On the walk around I noticed a significant dent on the airplane below the main cabin door. There was a dent sticker in the middle of the dent and to the right of the dent there was a jigsaw-like patch. There were also some other items that I did not like; including a sticky residue in the number one side of the hydraulics compartment and shavings on the number two side of the hydraulics compartment. There was no puddle on the ground; nor dripping from the compartment. It appeared to be residue versus a leak.On returning to the cockpit I looked at the hydraulics page and saw that the indications for quantity were in the green; and there appeared to be no unusual log book entries for hydraulic service. I asked my Captain to come and take a look at all three items. He did. He agreed that while the residue was unsightly; there were no apparent leaks; no fluid on the ground; nor on the air frame that would indicate leakage on either side of the hydraulics panel. I showed my Captain the dent to the right and beneath the main cabin door. We both noted the dent sticker. During the flight; passengers observed a loud noise coming from the middle of the aircraft. Many said the sound was loud and sounded like a loud whine. They said the sound was from the right of the aircraft from the floor. There was [another air carrier] First Officer who was commuting. The flight attendant put him on the phone to speak with us during the flight; so that he could tell us what he observed. He agreed that there was a louder than average cabin noise. He said he was uncertain of whether the noise was from the top of the aircraft; or from the floor of the aircraft; but he said it was from the middle of the plane.Upon arriving; we called contract maintenance so that they could inspect the cabin and see if there was a reason for the cabin noise. The two contract maintenance mechanics inspected the inside of the plane and the outside and when they saw the dent; they pushed on it and found that the entire frame beneath it was spongy. They grounded the plane. We concluded that while the dent was significant; that there was a sticker right in the middle of the dent. The biggest threat was that we rely on the dent sticker program. It is not a current; nor adequate; real time picture of what is happening to the airframe as it progresses through the system.I learned that although there is a sticker program; and although we rely on this program; it is not sufficient. We are taught that if there is a dent; and no sticker exists; we should phone maintenance and see if there is a record of a dent. If mx control says that they have a record of the dent; then we are to assume that the sticker came off through wear and tear. But it is that phrase 'wear and tear' that is ideal for this situation. The dent that we observed is right below where the jetway bridge attaches. Was it constant contact with a jetway that dented the airframe? At which station was it first observed? Did the dent that was originally observed; get worse? Was the dent re-inspected? When? Was the time for re-inspection sufficient; or insufficient? As operators; we don't know the answers to any of these questions.I think there is improvement that can be made with our dent program. We call if there is no sticker. If there is a sticker; we are to assume that there has been an inspection. If I had to suggest a solution; I would beg that we have a link on the website to go to and look at the dent; the sticker and the date last observed. When there is a dent observed and no sticker; we are asked to photograph the dent and email or text a photo. Could we have a photo library of dents; stickers and actions taken; planned or status available to pilots? That way; if we were to find a dent; we could look it up and compare it to the photo; the date observed; the dent as it was first noted; and then when we phone maintenance with new photos; we have relevant data to share with maintenance.We have our tablets and everyone has a smart phone; and we rely on the phone for our schedules; for changes; updates; etc. Even holdover times for deicing have been moved to an electronic file. If the dent program were moved to an electronic file; then we could simply communicate with maintenance much more clarity. A photo; along with the aircraft registration number; date taken; action taken; action planned - much like the log book that we inspect prior to flight; this electronic log would help us monitor; with up-to-date information; in order to get it right when it comes to airworthiness - and when the mechanics have to make a decision on action to take; or not to take.This is not a costly solution at all. As it is; we send photos back and forth. This would just be an orderly opportunity to catalog what happens to our precious assets; as they age and travel through the system. Thank you for listening; I would be happy to work on a solution with the appropriate department. The upkeep of our planes is vital; as they age; and having a chronicle of their upkeep is just one more way of taking care of the planes. Also; we could possibly track which station has a jetway that doesn't fit and help tailor a solution.




Synopsis: EMB-145 First Officer reported noting a dent under the main cabin door on walkaround.  During flight a louder than normal cabin noise levels led Maintenance to discover; post flight; that the dented area had underlying structural damage.