http://revach.net/article.php?id=4310

Section: Halacha   Category: Food
No Gesturing During Birchas Hamazon

The Shulchan Aruch writes (183:8) that there are those who say that the halachos regarding making an interruption by saying hello (out of respect or fear) during birchas hamazon is the same as during tefilah.  The Mishna Brura explains (183:30) that when the Shulchan Aruch refers to "tefilah" he is in fact referring to shmoneh esrei where the halachos are even more stringent than during krias shema and no interruptions are permitted.    

The Piskei Teshuvos writes (183:17) that based on this Mishna Brura, it seems that one can not make any of the interruptions that one may not make during shmoneh esrei.  Therefore, one should be careful not to even motion with their hands make noises to hint something or motion with their eyes during birchas hamazon just as one can not do these things while davening shmoneh esrei.