MAC Flapping is not easy to find; it is possible u have memory leak in switch; how old are other switches (other side of trunks)
Using MAC Vednor list I can tell you that one of MACs belongs to Intel, the other one to Quanta Computer -never heard of them; Sun Microsystems, USC; I was looking for MACs belonging to Cisco.
And...how do you know there is no loops? Just curious, most times I will ask end users, who claim they know nothing, and onsite I find some mini switch somewhere looped onto itself because 'the cable looked like it should go there' lol
Maybe if you can go onsite you can use bpduguard to resolve, shut down the ports who are seen bpdus when they shouldnt.