Probable issues are as follows :
1. improper grounding issue or broken/large ground loop,
2. you're using a non-regulated power supply or if the power supply is regulated, it is already starting to fail and no longer supplying adequate current.
3. Poor or unshielded guitar cable.
How to isolate the issue? Plug your guitar directly to an Amp and turn on the built in OD/Dist of the amp.
1. If the hum goes away, the power supply on your dirtboxes/effects is the culprit.
2. If the hum is still there, replace your guitar cable with a properly shielded guitar cable and if the hum goes away, then the culprit is the guitar cable.
3. If the hum is still there after you have replaced the guitar cable, most likely you have a poorly grounded pickups wiring like the ground loop is broken or you have poorly soldered ground loop.
HTH