Check the position of the mode selector switch. You may be receiving a SSB (single side band) transmission that is not on the exact same frequency as you or, your radio is in AM or FM mode. You will need to select either USB or LSB (upper or lower side band) and vary the clarifier knob until the sound becomes intelligible. If the sound does not become clear, try the other side band and repeat varying the clarifier knob.
Conversely, if you are in USB or LSB mode and there is a transmission on AM or FM, it will have the same garbled sound. Try changing the mode to AM and the FM. If it does not become clear, it is likely a SSB transmission and you are not on the exact same frequency of the transmission you are hearing. Vary the clarifier knob until it becomes clear - or change to the other side band if needed and try the clairfier again.
The short story is that you must be in the same mode and on the same exact frequency. When in AM or FM, this is less of a problem as these are much more wide banded signals than SSB signals.
The 2990 is really a 50 watt 10 meter amateur radio - but is geared to CBer's as it is easily modified to work in the CB frequency range and the so called "free band" between USA's CB (11 meters) and worldwide amateur 10 meter bands. Be careful transmitting this radio on CB frequencies as it violates FCC rules for power output there - and transmitting in the 28MHz - 29.7MHz requires a valid amateur radio license. Fifty watts on ham frequencies is tiny compared to the 1500 watts permitted. Why not get your ham license (here's how
) and leave the "Chicken Band" (and all it entails) behind. You won't miss it!
I hope this helps and good luck!