The talkgroups are less important than the actual voice frequencies used in this case. So note the voice frequencies that show up on your scanners' display when you hear the noise. Also, your description of the noise sounds like it could be from your scanner's overloading due to nearby NEXTEL sites and/or possibly other cellular telephone 800MHz sites. GRE-based designs are particularly prone to this; this includes many Radio Shack units (like the Pro-197 and the handheld equivalent that I can't currently remember the number of right now). You stated that you noted it on several scanners - what models were these? If any were Uniden models and you still heard the noise then that is very relevant as the Unidens are less prone (though certainly not immune) to this overloading issue.This noise is actually almost like a control channel type sound behind the transmissions. Very strange. I'll try to record it tomorrow. I'll also make note of what TG's I hear it on. It's intermittent, but I've heard it on a couple of EMS TG's, as well as Escondido Fire Dispatch.