I would suggest to try a factory reset on the DL4 by:
Holding the A and D footswitches and power up. The power is turned on and off on the DL4 by inserting a plug into the left mono input jack. The DL4 will need to have either battery or power supply power in order for it to be reset. If you did it correcltly you will see the Red LED lights of the DL4 light up from side to side. Try it again to see if the loop sound quality changes at all. One other thing to try it turning up the far right most knob on the DL4 that is labeled mix. This should turn up the loop volume playback, just in case the noise is related to the output setting or pot.
If you are still having poor sound quality with the DL4, you will need to have it assess by a authorized Line 6 Service Center. To find one in your area, follow the link below: