Hoping some pilot can help me out. I took my Phantom 4 out for a flight today and I heard a beeping noise before takeoff that I have never heard before. Everything was warmed up, props were turning, and no fault indicators on my flight screen, and nothing on my RC. I was about 5 feet away from aircraft and I just started hearing a repeating beeping noise. Could not see anything wrong anywhere so I took off hesitantly and let the aircraft hover about 6 feet high and about 10 feet from me. The sound went away and I continued with an uneventful 20 minute flight. Any idea of what the alarm was for?