Doh! I think that rules out the pulley wheels. My next guess is the chain ring bolts as you suggest, they are on the right side obviously so that aligns with the asymmetry as well. I had changed the chain ring at some point and might not have torqued them enough. The sound seemed to be coming further back but it is hard to pin down .. a rabbit hole indeed.

EDIT: chain ring bolts were surprisingly loose.. just tightened them and about to commute home to test..