Why would anyone trust an unidentified/anonymous witness who didn’t actually view any of the events, but claims to have heard what went on in the police van for all of 5 minutes of the approximately 30 minutes that Grey was in police custody in that van?

That’s witness testimony that while admissible as to what the witness heard, but claiming that the sounds heard were consistent with Gray engaging in self-harm is opinion and would not be admissible. The witness has no way of knowing exactly what was going on, who was in the van with Gray, and therefore his conclusion (that Gray was engaging in self-harm) is faulty.