Persistent Hum Across Amps, Cables, Guitars

[davidb]

I just moved into a new house and am suddenly encountering a lot of noise in my guitar rig. I've switched guitars, cables, amps, outlets, and no dice. This is with the guitar plugged directly in, no pedals. The noise drops significantly when I touch the metal hardware on my guitars, and I'm not so sure it is a guitar grounding specific issue since it is across my whole stash. Also many of my outlets do not have a proper ground and have a GFCI.

Any ideas on how to fix this? I'm not sure where to go next. Thank you in advance!

[acheld]

First thing I would do is check your outlets with an outlet tester. They are not expensive.

It's very common to see mis-wired outlets.

One thing to bear in mind is that outlet testers can not test GFCI without a ground -- but a GFCI device can work without a ground since it is comparing current though line and neutral. 

[davidb]

I’m already pretty sure that there isn’t a proper ground in some of the outlets according to the home inspection by the electrician. But it is worth a second check. I think it would be pretty expensive to get a proper ground installed. Would power conditioners or isolators work? That would be a lot cheaper than the electrical work, and it’s already safe to use thanks to the GFCIs

[bmccowan]

Do you know how old the house wiring is? Is the wiring two conductor cable - or does the cable have a bare ground wire?
I agree with acheld that all the outlets should be tested for polarity. I would also call the electrician that did the inspection and ask for more details.
GFCIs do not guarantee safety if there are wiring errors.