Shimano Nexus Inter-3 hub - rubbing noise ?

I searched the archives and came up empty so I'm hoping someone might have advice or suggestions on this.

The wife and I just bought a pair of new cruiser bikes made by Electra with Shimano Nexus Inter-3 hubs. On my wife's bike every now and then it makes a noise while riding that sounds almost like the chain rubbing on the tire yet no rubbing is occuring. I've put the bike on my stand and of course cannot duplicate the noise. The shop I purchased from looked, test rode, and also couldn't duplicate. I've readjusted the shift mechanism just to ensure it was correct but that made no change.

Thanks in advance for any help,

RJ