Mmm, I have some ideas but wouldn't say I know, I'm a with MJMP as it may be a strange artifact (as in it's not meant to happen, and normally wouldn't), (hey MJMP, is the signal stereo before the chorus i.e. are there 2 signal paths through it ?). Stereo (and mono) chorus is a combination of slight (very short <=40ms) delay and modulation of one of the 2 signal paths. This is designed (originally) to simulate a 12 string (each string pair is picked very slightly differently in time (delay) and each string modulates (spins) slightly in and out of tune with the other, modulation (of pitch)). In a mono unit the 2 paths are mixed back together, in a stereo unit (like MP1 chorus) the separate paths are fed to left and right outs (typically BTW as there's other ways to do it). So it seems your getting a resonance artifact (but I'm theorising here) in the B path. Maybe replace the chorus chip ? (MJMP ?).
Big question ? are you running stereo ? if not then A out will not necessarily have the delayed/modulated sound (which does vary in volume and pitch (modulation)), that's on the B out, combine the 2 A/B in stereo and you have stereo chorus and may answer the riddle ?