Sometimes old, cold, string wax sticks to the belly side of the curves below each limb, and as you draw it, you hear 'clicks' or 'pops' as the wax lets go of the string.
My Martin Rebel does this all the time, even after adding felt to the string groove on the limb belly. A tip that Rob Lee at Bob Lee Archery gives to quieten string slap upon release, and that will also stop the clicks & pops upon drawing, is to remove the string and soak the loops and up to about 7 inches of the string in mineral spirits for about 30 minutes, wipe off all the mucky wax with a paper towel (yes a little color comes off, too) and repeat about 2 or 3 times. Works great, quietens down the string and stops the pops. If it's not the wax, you've got problems. Thank Rob, not me.