Just curious. Have you put a scope on the power supply to check the AC component during transmit. A degraded filter cap or even a transistor with a small amount of leakage will generate what seems to be "a touch of white noise" under considerable load. Might seem too simple, but it happens alot. The RX will sound fine as not much load on supply. I'm assuming the TX freq is dead on? ( as you mention it is more prominent in some receivers than others) Did'nt see any mention of checking the obvious first.
Just a "kiss" thought. Best of luck!
