Default Vibration on earspeaker during calls.

Sometimes i have noticed vibrations on the earspeaker during calls and i have to put the volume down at least to 70% of the volume bar.
Strange that i have no problems with skype or hangouts calls. And also I have try an app called earpiece to use the earspeaker as the main speaker to reproduce music, and all OK.

So it seems my problem is only during calls. Can someone help me? Thanks

Devices
: Nokia 6510 -> Sony Ericsson Z600 -> Sony Ericsson W880 -> Sony Ericsson W980 -> Sony Ericsson Xperia Arc -> Google Nexus 5 32GB