Deleting microphone whine

Hi
I am using Windows 10 and Audacity 2.3.2.

I have a video via my Sony Handicam of 4 men singing with recorded music.

In the beginning of one song there was apparently a resonance btw one microphone and another one resulting in a tone increasing in value before a sudden drop off as the next part of the song came along.

In the attached screen shot if you look in the shaded box section you can see the edges of the inner lighter coloured section increasing in width in both bands.

Could someone suggest how I might make those edges horizontal? Thank you.
FourWhine.jpg

Envelope tool can do that …

But to fix resonance Audcaity’s spectral editing tools are ideal.