Arranging pedals for the best results/lowest noise is a black art; but if it were me, I'd take the BOSS tuner out of the chain (I mean, honestly, how often do you have to retune in the middle of a song).
If I'm understanding this right, you may be boosting gain THREE times (BB into GS into T21): IF that's right, and it then goes up the snake to a board where it's maybe not trimmed correctly in the channel (more heat!), it's a wonder you don't hear the hinge in the pedals squeaking! Even if these things are providing only a moderate gain, several in series adds up quick. A painless check for this would be to switch everything in, and if you can hear the chrous sweeping with no notes being played . . . well, I'd think about doing some swapping out to find what you can do without.
And like hieronymous said above, there's no telling what is/isn't bypassing, maybe there's a bogus switch or jack, etc. You'll just have to swap stuff in and out, change the order, etc., till you figure it out.
Generally, you distort first, then add the delay/modulation effect afterward. This way you're only boosting the noise 'up front' and not boosting all the random buildup that's in front of it. I prefer to leave tuners out of the chain to eliminate any more noise or one more modifier of my original tone (for me, I'd NEVER play thru a tuner).
Good Luck,
J o e y
