I'm using a zoom 505ii guitar pedal and it's my main pedal for my sound. I've never had a problem using it with my delay & wah pedal before it, but once I started connecting more pedals to build a pedalboard it started making a weird humming noise or squeal. So I'm lost in where my pedal should be, At the end or mixed somewhere in between?
As far as order of effects you generally want something like this:
compression > OD/distortion > modulation (chorus/flange/phaser) > delay/reverb
Wah can go before or after OD, you'll get a different sound each way. This is the way a lot of people do it, but you can get different sounds by changing up the order so do what sounds best to you.
Your order depends on how you use your multi-pedal. Usually multi-effects pedals are better at modulation or delay than they are at OD/distortion, so generally I use standalone OD/dist pedals and I would put those before a multi-effects unit in my chain (OD before modulation).
If you are getting noises/squealing that might be a bad cable or connection somewhere, or even a bad battery or faulty power supply. Try to troubleshoot your pedals one by one and see if you can find which ones are making the most noise. If the multi has a noise gate that would be a good reason to put it toward the end of your chain, and it could potentially get rid of some of the humming.
Do you have an effects loop in the amp? That would be a good place to put modulation and delay/reverb effects, and keep the OD/dist pedals before the amp. Let us know how you are using your multi pedal and what other pedals you use with it.
The way I look at it is GMT. Gain, Modulation and Time before the amp. (With variation. Wah first, generally, even if it is modulation.)
I have a more substantial multi-effect unit, which I use for reverb, echo and amp emulation, and a pedalboard with compression, several dirt pedals, a clean boost ("more!"), an attenuator ("less!") and a volume pedal ("aw, swell"), and I put the pedalboard before the multi-effect unit.
But your mileage may vary.
I have exactly the same zoom 505ii box (they have a brilliant built-in tuner!), and noticed it started making a digital hum kind of noise. Is that what you mean ?
For me, the issue turned out to be a badly shielded cable (ie, an old one where the internal shielding was letting interference in), from guitar to effects board.
This was doubly weird because my effects board just connects to the zoom 505ii to use it as a tuner. I didn't have the output from the zoom 505ii connected to anything, making it a "dead end". So it must be emitting some electromagnetic stuff in its normal running which a seemingly disconected wire is picking up.
It might be worth checking the patch cables you're using, maybe swap some about to see if any are more noisy than others ? It;s certainly a place to start.
However if this isn't the issue, it "shouldn't" really matter in terms of hum what the order is, but effects which 'bring up' the hum are distortion/overdrive and compression. If there's no hum introduced before them, they'll have nothing to work on so maybe these should go first in the loop.
The zoom 505ii has a built in settable noise supressor so you could argue that if you were to use that, you should put that last in the loop and set the noise supppressor accordingly, so that it squashes any background hum from the rest of the setup.
Something like .. Guitar->compression->distortion->zoom->(anything other than comp/distortion)->amp