Murmering

My wife continues to report the murmering that can be heard in her store. She cannot track it down. She says that when she is not busy, she has gone outside, walked all around her store and nothing is heard except traffic on the street and quite often there isn't that much traffic. The sounds are noticeable only in the store, not all the time, but frequently. I go to the store in late afternoon to help her close. So far I have not heard the murmering there. She says the murmering cannot be pinpointed, it seems the same all over, whether she is in the main showroom, the back showroom, the work and stock area or the bathroom. I understand her problem as I have experienced the same sort of phenonema here at our house; just not nearly as often. I may have her take a recorder and see if we can record the sounds.