Buzz In Chromacolor II Sound- 25GC45

My sincere thanks to those who helped me solve my horizontal output problem on this set. For the sake of perfection, I'd like to solve a less serious problem that has been plaguing me for awhile.
I have a standard definition satellite feed (directv) going into channel 3 via RF. When the picture has bright white areas (particularly annoying on the news networks when there are constant banners at the bottom of the screen) there is an annoying background buzz/hum, as though there is video leaking into the audio or the signal overloads the set. (I tend to discount the latter because some channels, particularly the local ones, seem better than others.) Fine tuning doesn't help because when the picture is optimal, the sound is at max. volume but so is the buzz. There is no problem at all when the screen material has moderate contrast throughout.
Could someone please offer advice here? I should point out that I replaced the sound board on this set recently, and the buzz seemed worse afterwards.
There may always have been a little but not nearly so noticeable.
The chassis on this set is a 25GC45/45Z.
As usual, help much appreciated!
Frank