Noise is 'routinely' blocked inside the metal (power supply)
box by the line filter - if filter exists. You are fooling
yourself if you think some coil on the power cord or grounding
is going to solve the problem. The power supply appears to be
defective when designed. It appears to be another classic
example of power supplies designed for MBAs types who call
themselves 'computer literate'. Overseas manufacturers have
discovered a lucrative market of technically naive computer
'experts' AND have dumped product into that market for higher
profits.
You don't need a local electronics store. The internet is
chock full of responsible vendors who sell minimally
acceptable supplies. Supplies that meet Intel ATX power
supply specs - and say so. Supplies that meet FCC Part 15 and
the even more stringent European standards. Supplies that
must also meet ISO, CE, UL, and other standards - all in the
same box. Where are those approval labels on your power
supply? What other problems will those missing standard
(functions) create?
A previous post listed other functions that the power supply
must provide. For example, does that supply have over voltage
protection (OVP)? If not, then a power supply problem could
destroy every other component inside a computer - later. OVP
was standard even 30+ years ago so that a power supply failure
to 'never' damage other computer parts. And yet power
supplies missing essential functions such as AC line filter
would also 'forget' to include other important functions.
How would you know if that supply has OVP? As pbdelete
noted - open the power supply to identify an AC line filter:
Make sure there is a line filter. Like those with two
inductors in serial to the mains.
If no filter, then you are not going to fix that supply.
Other essential functions, such as OVP, are also probably
missing. A supply designed for bean counters - people who are
so often the reasons for failure.
Noted earlier was how a power supply sells at lower price
for greater profits. OVP is but another function routinely
missing on power supplies designed for bean counter
consumption. Power supply that provides no written
specifications because so many such 'computer experts' only
look at two numbers - dollars and watts. People who will
rationalize their decision rather than take hard and
conclusive actions.
If that power supply does not have an AC line filter as
pbdelete asked to inspect for, then that missing function may
be part of an iceberg. Your only viable solution is a new
power supply from a responsible power supply vendor and
manufacturer. Some computer parts houses don't even stock
such supplies.
Meanwhile, what is accomplished by learning AC house
wiring? Learn which wires are better and worse transmitting
antennas. Again - and it should be so painfully obvious - no
grounding was going to solve your RFI. That ground wire
inside the wall only becomes a transmitting antenna - as was
stated previously.
There is no plug-in or power cord solution for a missing
filter that was required and standard even generations ago.
There is no "coil to put around the the AC cord". A supply is
causing RFI interference. Then the entire supply is detective
by design. Completely unacceptable, a reason for more future
problems, and a classic example of why people with MBA
mentalities create so many failures. Patching a solution on
the power cord is called 'curing symptoms' or a 'kludge'. A
solution must exist at the problem - inside that power supply
box - where the solution would have cost so little money.