Date: 201808
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: EMB ERJ 170/175 ER/LR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: Taxi
Result: Aircraft Equipment Problem Dissipated; Flight Crew Returned To Gate
Contributing factors: Aircraft; Environment - Non Weather Related; Human Factors
Primary problem: Environment - Non Weather Related

Narrative: After taxing to [to the runway]; Ground Control initially advised there was no ground delay program for [out destination airport]. With that information; we had decided to start both engines to prepare for an immediate takeoff. However; shortly thereafter; Ground advised [our destination airport] just went into a ground stop and instructed us to taxi into the run-up pad on the north side of Runway XX. Ground advised the stop would be at least 35 minutes. After taxiing into the run-up pad; we restarted the APU and shutdown both engines.Shortly after engine shutdown; a very loud banging sound was coming from the floor of the airplane. At first; I had simply assumed it was my A Flight Attendant simply banging on his ice in the galley cart. However; my First Officer also was concerned at what the source of the banging noise was. This banging was rhythmic and not normal. As I prepared to call my A Flight Attendant to inquire what they were doing back there; the A Flight Attendant was already calling me to ask the same thing as he was concerned too.As we discussed the source of the sound; it grew louder at this point. My FO offered up the idea that perhaps that there's possibly a ramp employee trapped in the cargo bin trying to get our attention. The sound was in fact very similar to times where I've heard ramp personal banging a beat the on wall as they await bags. Having heard of similar incidents in the industry; this wasn't a very far-fetched idea. Since we were on a ground stop; we elected it would be best to have this investigated immediately.As we started the engine to taxi; the sound become more rhythmic; almost to the point it was signaling an SOS; dot dot dot dash dash dash dot dot dot.  It was so pronounced I even stomped my foot once in return; my FO chuckled and said amongst ourselves 'we hear you buddy.' The A Flight Attendant still on the inter phone advised that he possibly heard a muffled yell.We returned to the gate and advised the ramp personal to open all the cargo bins and search them. To our shock there was nothing in there but baggage. Disturbed and startled at the source of the banging; we advised Dispatch and Maintenance to inspect the aircraft. A theory was then produced that the source of sound could've possibly been from the nose wheel steering mechanism. We deplaned the passengers and conducted a high speed taxi; parked the aircraft; shutdown the engines to reproduce the exact scenario that we encountered the sound and we heard nothing but silence. Having no evidence of any source of the sound; we returned to the gate; re-planed the passengers; Maintenance signed off on the aircraft; and we departed without further event.




Synopsis: E175 First Officer reported after engine shutdown; a loud banging noise was heard coming from the cargo pit area.