iTunes sync "beep" loop iPhone and mac

About half the time i plug in my iPhone 6 to my MBP2015 15" to sync and they get stuck in a loop: the phone will beep to indicate the start of a sync, and the screen flashes, then 1 second later it beeps again. It keeps beeping until unplugged. Fully rebooting both devices at the same time seems to work, but its a pain... Rebooting either is not effective: they both need to be booted if the loop has happened.

Any tips on how I could further debug? Extract from console logs below for two "beep" cycles.

iOS 9.2.1 (13D15), OSX 10.11.3. All the latest updates installed.

10/02/16 8:35:55.972 AM iTunes[926]: Entered:_AMMuxedVersion2DeviceConnected, mux-device:80
10/02/16 8:35:55.974 AM iTunes[926]: tid:4217 - unable to query device capabilities
10/02/16 8:35:56.084 AM iTunes[926]: Entered:_AMMuxedDeviceDisconnected, mux-device:80
10/02/16 8:35:56.084 AM iTunes[926]: Entered:__thr_AMMuxedDeviceDisconnected, mux-device:80
10/02/16 8:35:56.084 AM iTunes[926]: tid:4217 - BootedOS mode device disconnected
10/02/16 8:35:56.084 AM ath[978]: _receive_message (thread 0x7fff7f61a000): Far end closed connection: Resource temporarily unavailable
10/02/16 8:35:56.084 AM AppleMobileBackup[1034]: lockssl_handshake (thread 0x7fff7f61a000): SSL handshake controlled failure 0: SSL_ERROR_SYSCALL (Early EOF reached)
10/02/16 8:35:56.085 AM iTunes[926]: _receive_message (thread 0x700000cdc000): Far end closed connection: Undefined error: 0
10/02/16 8:35:56.085 AM ath[978]: AMDeviceStartSession (thread 0x7fff7f61a000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.085 AM AppleMobileBackup[1034]: AMDeviceStartSession (thread 0x7fff7f61a000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.085 AM AppleMobileBackup[1034]: 1034:2137104384|DeviceLinkListener.c:_copyMobileDeviceValue| ERROR: Attempt to copy a value from an unpaired device. Cowardly refusing to do so.
10/02/16 8:35:56.086 AM iTunes[926]: AMDeviceStartSession (thread 0x700000cdc000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.086 AM iTunes[926]: failed to connect to USB device 0x7fc076146350, UDID 9a86d929e12afd57eb327ba220b13536a4ce8269 (0xE8000084)
10/02/16 8:35:56.092 AM AppleMobileBackup[1034]: INFO: Device detached 0x7fa303d00b80/9a86d929e12afd57eb327ba220b13536a4ce8269
10/02/16 8:35:56.414 AM iTunes[926]: Entered:_AMMuxedVersion2DeviceConnected, mux-device:81
10/02/16 8:35:56.421 AM iTunes[926]: tid:14f2b - unable to query device capabilities
10/02/16 8:35:56.605 AM iTunes[926]: Entered:_AMMuxedDeviceDisconnected, mux-device:81
10/02/16 8:35:56.605 AM iTunes[926]: Entered:__thr_AMMuxedDeviceDisconnected, mux-device:81
10/02/16 8:35:56.605 AM ath[978]: lockssl_handshake (thread 0x7fff7f61a000): SSL handshake controlled failure 0: SSL_ERROR_SYSCALL (Early EOF reached)
10/02/16 8:35:56.606 AM AppleMobileBackup[1034]: lockssl_handshake (thread 0x7fff7f61a000): SSL handshake controlled failure 0: SSL_ERROR_SYSCALL (Early EOF reached)
10/02/16 8:35:56.606 AM iTunes[926]: lockssl_handshake (thread 0x700000cdc000): SSL handshake controlled failure 0: SSL_ERROR_SYSCALL (Early EOF reached)
10/02/16 8:35:56.606 AM iTunes[926]: tid:4217 - BootedOS mode device disconnected
10/02/16 8:35:56.606 AM ath[978]: AMDeviceStartSession (thread 0x7fff7f61a000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.606 AM AppleMobileBackup[1034]: AMDeviceStartSession (thread 0x7fff7f61a000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.606 AM AppleMobileBackup[1034]: 1034:2137104384|DeviceLinkListener.c:_copyMobileDeviceValue| ERROR: Could not start session with device: This device is no longer connected. (132)
10/02/16 8:35:56.606 AM iTunes[926]: AMDeviceStartSession (thread 0x700000cdc000): Could not start session with device 9a86d929e12afd57eb327ba220b13536a4ce8269: kAMDDeviceDisconnectedError
10/02/16 8:35:56.607 AM iTunes[926]: failed to connect to USB device 0x7fc070e52cc0, UDID 9a86d929e12afd57eb327ba220b13536a4ce8269 (0xE8000084)
10/02/16 8:35:56.607 AM AppleMobileBackup[1034]: INFO: Device detached 0x7fa303ea61b0/9a86d929e12afd57eb327ba220b13536a4ce8269

The answer is simple: Your iPhone needs to be charged at least 50% in order to sync in iTunes. (My iPhone was charged at 45% and I got the dreaded beeping sound today. Absurd? Yes.) Once you charge it more fully, the beeping should cease. At least it did for me.