Have you done the obvious thing and pulled the pedals out of the chain one at a time to see what stops the noise?

Putting the Big Muff in the effects loop would be unusual. It would be after the preamp and after the other effects either series or parallel. A more typical configuration would be putting the OC2 and the DD5 in the loop. I guess it is a way to provide true bypass, but I'm not sure it would sound the way you want.

For starters, the Big Muff is notorious for noise issues. Of course we don't know if your "noise" is "hiss" or "hum" or a combination of them. Maybe you can describe the noise(s) you are getting. Are you using batteries or a power adapter; if an adapter, which one? For example, Boss pedals don't like certain power adapters and generate a bit of 60-hz hum with them.
Why are you using both the Green Screamer and the Bif Mugg? Do you use them at the same time? If so, that could be your problem right there.
Now, a little bit of signal chain tutoring: a tuner should probably be the first "pedal" your guitar sees. All you have is the pick-up to the tuner, so you tend to get better accuracy from the tuner that way. Then the OC2 octaver, once again, it tracks the clean pick-up signal better than a fuzzed signal. Besides, a "fuzzed octave" sounds better than an "octaved fuzz", if you know what I mean. Than the Russian Rat (Big Muff), the Screen Creamer, the DD5 delay last, then the amp.I wouldn't try the effects loop on the amp for the Muff. That loop is a boosted line level signal which is too strong for the Muff. This will squash it and make it even noisier.

BobRodgers>>

Yes, and it looks like the problem is with the Big Muff.
Thanks for the info on the effects loop on the amp.
I just saw some modification information on the web to change
the Big Muff into direct bypass.

Moonbaby>>

It's a bit difficult to describe the noise, but it more a "bzz" than a "hiss/hum".
All of my pedals are powered by the Godlyke Powerall.
At first I thought it was the Powerall, but after changing to a battery setup I still had the same bzz so it wasn't that, although it could be contributing.

The Big Muff and Green Screamer are never on together, it's either or.

But I'll definitely play around with the sequence to see if I can get rid of some noise.

OK. BTW, if you try using the amps effects loop with any of these pedals (although that loop was really designed for rack-mounted type effects units), you'll probably want the switch set to "parallel". "Series" interrupts the signal chain between the preamp and power amp sections of the guitar amp. This is OK for an equalizer or a compressor. "Parallel" doesn't interrupt that flow, it simply runs along with it, more like the Aux Send and return on a mixing board. And as to the noise, a more "buzz-like" irritation may indicate a faulty ground connection, either a funky cable or a funkier Mig Buff...

Given your gear, the primary suspect would be the Big Muff. Those of us who were around in the 70s remember that the Big Muff Pi was the cheapo fuzz box that kids used (because of the price). It sounded like a box of bumble bees. Why they ressurrected it is beyond me. I guess it's the "vintage" appeal. LMAO

I've always been a stompbox kinda guy. I bought a Korg AX3000G a while back, and I really like it. It's replaced several of my stompboxes. For $250, I think it's a steal. You may want to try one!