Your strings should be grounded as David L said and they can be subject to RF interference from fluorescent lights,computers,fans, electrical wiring and switches in your house. Non shielded wiring in your git can be a problem as well as feedback from the space in the box. Bad cables or amp problems can also cause hum.
Regardless of all that, Single coil pickups give 60 cycle hum(noise) unless you have a noiseless pickup. So you'll have some hum to put up with regardless. There are ways around that, but it's involved.
Some things you can do to prevent problems.
Line the inside of your box with aluminum tape or copper shielding tape.
Twist your + and - wires to create a shield or replace with shielded wires.
Put a piece of double sided tape across the bottom of your pickup(cheap way to help with feedback).
Get rid of hum by installing a dummy coil.(Wiring another pickup that's rwrp of the other in series and has the magnets removed)
No problem. Ask away if you need help or suggestions.
Glad to be of help.