Mark:
Untie the therocouple lead from the AC line and take it up to the seam along the top of the firewall and slip it through the harness clips along that seam. Don't secure it tightly. See how that works. The thermocouple lead is braided and may be transferring the engine noise to the AC line and amping it up so you can hear it. Remember in the 6.2s and 5s the injector lines had to be cleared or you would swear that a lifter was clattering. Same thing could be happening here. No gaurentees though.
Denny