Check the input signal. The 303’s unbalanced inputs are designed to run at -10dBV nominal, and the 303’s balanced inputs are designed to run at +4dBu nominal. Make sure that you are feeding a line level (not mic level) signal to the 303 at the appropriate nominal level. Check gain settings on downstream equipment. The system gain structure should be such that the 303 operates at or near unity gain. Is the input signal already noisy?