Chrome or metal tape is playing. NORMAL bias tape is recommended for best sound quality.
The deck mechanism of the unit could be dirty. Carry out the cleaning procedure by first, slightly moisten a cotton swab with alcohol or head cleaning fluid, then press PLAY to clean the rubber pressure rollers with the swab. After that press PAUSE and clean capstan and the head. Cleaning tape may also be used for Head cleaning.
Low battery. Connect to AC power or insert new batteries and check unit for normal operation.