A thought experiment/analogy:

Think of  AGC-T as trying to siphon water out of a bucket, A bucket with an inch of mud in the bottom. only insert the siphon hose deep enough into the bucket to siphon the water, think of the water as signals, and think of the mud as noise. think of the threshold setting as a way to control the depth of the intake of the siphon hose. 

Then remember that that the slider operation is reversed in function the higher you set it, the closer to the mud you will be, and the more mud your going to be siphoning out.

The lower you set the AGC-T slider. the cleaner the water will be. you want it inserted to a depth that just above the mud, but not in it. The point of finding the knee in the AGC-T action is to get as close to the mud as you can without siphoning mud.

You may find out that the knee is actually lower than you'd guess it to be. Above the set threshold the AGC will act on the signals to even them out in volume, the passband audio below the threshold won't be passing through the AGC. IOW it's excluded.

Finding the knee will get you in the ballpark, if you still hear noise, try dropping the AGC-T slider setting a 1 or 2 % until the noise is less audible.

If some of the signals seem overly loud set the slice volume lower.

I realize this is a very imperfect analogy, but it's how I think about it.