I may not have an answer to your question but I have some healthy advice for
you. I worked in electricity transmission and distribution for the last 15
years, got within the limits of approach at voltage level going from 120V to
750kV on two continents. My main specialty is work safety so what I'm
telling you is mostly from this perspective.
Remember that the ONLY protection you have, as a living creature is the
ground (what you call "earthing"). If that fails you're virtually dead -
meaning that you're not breathing, seeing, talking, eating, f..., etc - the
fact that didn't happen yet doesn't mean it cannot happen, just that it
didn't happen yet. If you have any doubt, even the smallest one, that your
house grounding is not 100% safe have it checked ASAP before what I just
said happens.
As a rule of thumb, for normal people like you and me (and probably 99% of
the guys writing here) everything above 24V is lethal and should be treated
accordingly. There's only one thing that makes the difference between life
and death: your skin resistance. As long as that holds you're ok, as soon as
it fails you're gone. What's underneath your skin is mostly salted water
which guarantees a massive failure due to the electrical current passing
through. Examples of how your skin resistance can decrease are: moisture
(sweat, hands recently washed, hand lotion, etc), cuts, burns, scratches,
etc. In other words, there is one thing that works for you, one million
things against you.
Household grounding failure is a main cause for many accidents, some lethal,
some not, some with long term consequences. Remember that copper oxidizes,
the oxide increases the contact resistance which diminishes the efficiency
of the ground connection. Also contact pressure has a tendency of decreasing
in time with the same effect. Also bending the solid copper wires can reduce
the copper section increasing the resistance thou reducing the overall
grounding efficiency. In this scenario, one hand on the metal case of your
PC and the other hand on the floor establishes a current path dangerously
close to your heart, you had no problem when you first tested it because
your hands are dry, next time you'll try it after eating an orange, you'll
have some juice still on your finger and the next person entering your room
will find you immobilized there and if this happens after more than 4
minutes your brain already suffered irreversible damage... Even if they
manage to re-start your heart they'll never manage to fully re-load your
operating system...
Again: if you have the smallest suspicion that your house grounding may be
faulty or not necessarily faulty, just if you thing it lost some of it's
efficiency, have it checked. If you know exactly what you're doing you can
do it yourself.... which I believe you would've done already... if you knew
what you're doing... remember that money are of no value if you're not
around to spend them, hire a certified electrician to have your grounding
checked and eventually brought back within proper operating parameters
before something unwanted happens.
This may also fix your problem... but again, it may not. The switching power
supplies used in computers are converting the household AC voltage into DC,
converts it back to AC but this time with a high frequency that allows use
of a little high efficiency step down transformer to reduce the voltage.
Once reduced it's converted to DC that's used by all components. To
eliminate - as much as possible - interferences between the household AC
frequency and whatever goes out of the power supply they use all sorts of
filters (capacitors, inductors, etc). As you know, all filters have a
certain range where their efficiency is high, outside of that range the
efficiency decreases so it may allow some system frequency components or
harmonics to enter your PC. If the ground is ok it may be that you, acting
as a capacitor, change the filter range a little, sufficiently to reduce the
hum to more acceptable levels. In which case you may want to try a different
power supply, one that may better filter the primary components.
...sm
