Clicking noise before message alert

Hello guys!

I own an iPhone 5 which shows a defined clicking noise (no cam-lens rattling etc.) before a message alert arrives. This noise can be reproduced as well and can be heard well in a distance about 50cm. Its loudness is similar to the standard message tone.

Premises:

1. Vibration AND LED alerts have to be enabled simultaneously

2. It only occurs before a message arrives, incoming voice calls are unconcerned

3. use of the cam with LED flash on produces also no click

4. Vibration standalone or LED alert standalone produces also no click

5. Message tone on/off does no difference

Any ideas?

Cheers, Chris