Ok I've been reading this thread and it sounds like there are some vehicles that have some significant noise coming out of them. I think if the people who have the problem state the exact configuration of their vehicle it would be easier to diagnose what is going on. There isn't enough info in this thread to see a common theme....
Is it possible there is some sort of switching power supply in the vehicle? If you add to the electrical load does it get worse/better? (eg turning on all the lights, high beams dome lights - not the stereo though since that would mask the sound levels).
Don't discount an exhaust leak either. They can make all sorts of weird and crazy sounds including whistling, screaching, etc.
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'09 370Z - '93 300ZX Vert - '83 280ZX - '83 280ZXT -'81 280ZXT stroker - '72 240Z - Stillen CBE, HFC, G3, Grounding Kit, Sways // Swift Spring // H&R 15mm // 5% Tint // Clear Bra // 
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