Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Fatigue; Situational Awareness
Aircraft Equipment Problem Less Severe
Flight Crew Became Reoriented; Flight Crew Executed Go Around / Missed Approach
Aircraft; Human Factors
Entire flight was normal. On approach; about a minute after lowering the gear; we heard a loud 'thud' in the lower underside of the fuselage. My first thought was a blown nose gear tire or a large bird strike. Several seconds later; we heard and felt a very high frequency vibration. We were not sure what this was. FO was pilot flying. He initiated a go around. I agreed with his decision. We had no indications of any kind on EICAS. Still concerned we had a blown out nose tire; I [advised] Tower. A couple of minutes into this go around; I realized it was the ADG making this loud 'whirring' sound and high frequency vibration. The landing was uneventful. Maintenance and flight control were contacted immediately after parking at the gate.  Fire department and airport operations personnel met us at the gate. They interviewed me for their records. No undesired aircraft state. Fatigue could be an issue. This event occurred during the last leg of a long and stressful day. We had two other flight control issues earlier in the day that delayed us quite a bit. With a tight connection for a deadhead flight to ZZZ1 to pick up our overnight. I have ideas but this is not the forum to voice them. This may sound like a complaint; but it is not a complaint. Earlier in the day I spoke with scheduling; they were rerouting us to a DH and I voiced my opinion that would be really close and may not work out. I don't feel they wanted to listen. Communications involves both parties listening to each other. Again; this is not a complaint; just an observation.
CRJ-200 Captain reported executing a go-around when the flight crew heard a thump and noticed some vibration that was later traced to the Air Driven Generator. Reporter cited fatigue as contributing.