The only thing i can think of is try binding the RX to the TX using the Lipo battery and the 7.4v battery or just the the lipo it could be the signal is week. is your TX fully charged? I read this in the manual of the ESC i'm using on my Trex 450, i don't know about yours is like this it could be the reason for the motor beeping

If the aircraft should land or crash in an unexpected location and become lost, the pilot can enable the Aircraft Locator Option. The Aircraft Locator Option is engaged by tuming off the transmitter. When the ESC does not receive a signal from the transmitter for 30 seconds, it will start to send an alarm to the motor. The sound of the alarm will aid the pilot to locate the aircraft. This option will not work with a PCM receiver that has SAVE function enabled, or with low noise resistant PM receivers.

Rich