[Electronic M]

The text buzz issue comes from an internal design flaw in the RF modulator it's self, which likely*# can't be corrected by an outboard device. In am AM modulator traditionally modulation is to be kept less than 100% so the carrier is always on regardless of modulation, but if allowed to equal 100% the carrier is extinguished, and if it goes over 100% mod the carrier polarity reverses (creating a 180 degree phase shift).
Consumer modulators have a tendency to use the same carrier for AM and FM components, so if the AM video mod kills the carrier or shifts it 180 degrees in phase, the FM mod will still try to modulate what it is given resulting in noise added to the audio (drop out noise from 100% mod, and likely more critical PHASE distortion of the FM signal from >100% AM mod). Now a key here is that over modulated AM (from the VIDEO signal) creates phase modulation...Phase modulation and frequency modulation are nearly the same thing* and can both be received or transmitted with equipment designed for the other one with a simple signal processor inserted outside the RF system, so the AM modulator is interfering with the FM sound (via PM noise) every time the whiteness of the video gets strong enough to achieve 100%+ modulation (the white level in video corresponds the lower troughs of the signal so when those parts of the modulator envelope dip below 0V carrier on bright text or other bright objects the sud hits the fan). Outboard RF attenuation will do nothing to stop this since it has nothing to do with the level of the outputted RF, but instead the internal modulation signal level relative to the RF carrier before entering the modulator...You could attenuate the RF output to a hairs breath away from snowy reception and the buzz would still remain from the crap happening in the modulator.

A way to verify this (which I've done) is to take an old outboard Closed caption demodulator which has caption brightness and contrast user controls and feed the video output to a modulator you want to test. Start with the brightness/contrast at max and turn it down until the audio buzz, with a normal/dark program on, goes away.

*# It may be possible on some consumer RF mods to attenuate the video line in to them and or add a small DC offset to kill the buzz, but if there is any internal auto gain level circuits and or DC blocking circuits this will be useless.
The best solution is to get a cheap older pro-grade RF modulator with audio and video level controls to be able to properly set them your selves, and not have to worry about all the other (inexcusably)crappy approximations of the NTSC specs that the consumer modulators make.

*Phase mod and frequency mod circuits can produce the opposite type of output simply by having the input signal pass through an integrator or diferentiator before the modulator or after the detector (the specifics depend on what type the hardware is VS. desired mode of operation and whether you are on the receiver to transmitter end).