Weird results

I placed a sound activated tape recorder on top of a tall display case in our hall way right beside the wall where these short blasts of speaking seem to be originating. I left the recorder there for several days. This area of our house is rarely used by us. It is a section of hall yjat runs parellel to the front stair case and goes past the western door of the master bedroom and dead ends at the trunkroom (where Christmas decorations are stored. When I went to get the recorder it was not where I put it. I could not find it. It had not fallen anywhere around the area. I aasked my wife about it. She assured me she hadn't even been in that area of the house. I returned upstairs and did a double check of everything. I found nothing so in discust I turned and walked down the hall yoward yjer master bedroom; inyending to return downstairs. As I passed the door of the trunkroom I saw the recorfer sitting on a box of Christmas decorations. Total surprise. That box of decorations was a good 15 ft from where I placed the recorder. I saw the tape had run out so I placed new batteries inn the recorder and rewound the tape. When I played the tape the first quarter of it was blank or had distant voices which I determined were my wife and I and someone at our front door. So that was explained. Then the tape set up a buzzing for the rest of the tape. Just buzzing. Testing the recorder I found it works just fine. So whats with the buzzing?