The noise is not, but the cause of the noise may be.
Typically, such noise is caused by poor grounding and/or a "difference in potential" in the supplied voltages. And this is not uncommon when using two power supplies. Your dual PSU setup should definitely be considered a temporary solution until your budget allows you to buy a single PSU of adequate size.
In the meantime, you need to make sure you have one common ground throughout your whole system. So get some simple 22awg or larger (smaller number) copper wire. Run a ground from one the 4 screws currently holding the PSU to the case to one of the 4 mounting screw locations on the back of the second PSU. That alone may (if lucky) get rid of your noise. If still present, there is probably nothing else you can do because it is likely caused by slight difference in the supplied voltages of the two PSUs.
For example, the allowed variances are ±5%. So, for example, with your 12V, one supply may be outputting up to 12.6V while the second may be outputting as low as 11.4V. Individually, both supplies are within allowed tolerances. But when combined, the 1.2V difference is a full 10% difference. In the short term, this should not be a problem. In the long term, it may put increased stress (and heat) on the regulation components, increasing their aging.
Both supplies, and the monitor, should be plugged into the same wall outlet too. This ensures both supplies are connected to the same facility ground. Even if two wall outlets in the same room are on the same circuit, the path to Earth ground will be different lengths and therefore different resistance. That is not ideal when trying to minimize unwanted noise. It is not good for the best safety practice either.
And to the point of Earth ground, every home and every computer user should have access to a AC Outlet Tester
to ensure your outlet is properly wired and grounded
. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets (outlets near water) too. These testers can be found for your type and voltage outlet, foreign or domestic, (like this one
for the UK) at most home improvement stores, or even the electrical department at Wal-Mart. Use it to test all the outlets in the home and if a fault is shown, have it fixed by a qualified electrician.
Another cause of this noise could be interference, or lack of interference suppression. The grounding may help with that. And making sure your wall outlet is properly grounded may too.
Note that analog signal are much more susceptible to EMI/RFI
. So going all digital is probably better. Sadly, retiring perfectly good, but obsolete electronics before they die is just a fact of life. It may be time to let that analog monitor go.
But in the meantime, make sure the cables are securely fastened on both ends and in perfect condition. Try swapping cables and go as short as possible.
One last thing you might try is powering your fans and drives with one supply and your motherboard and graphics card with the other.