weird humming fan..

[prod]

I have a 43cfm Sunon in the back of my main machine. It makes a faint high-pitched humming noise ... when the power is OFF :beatsme:
When I move the fan with my finger, the noise goes away, but when the fan comes to stop, the noise comes back. When I turn off the switch on the back of the power supply, the noise stops.

When the power is soft-off, there should be nothing on the 12v line, correct? This a 3 month old Enermax p/s so I would be very surprised if there was a problem with it. And any kind of loose connection in the fan shouldnt make a difference with the power off. I suppose it could be the motherboard, as the fan is plugged into a 3pin connector. Anybody see a reason for this? What would be the best way to test for the problem?

[Cool Canuck]

I have notice whilst ( that's the way they like to converse around here :) ) working on my computer, while the computer is off and I disconnect the power the fans will relax. The blades, although not turning will move back slightly after the power is disconnected. It would appear there is a very small amount of power being deliver whilst the computer is off. I gotta use that word more often. :hat:

[frombadtraverse]

I do believe that all power suplys provide a certain 'standby' current. Could be following a leakage path to the fan. Have you tried plugging it into another header?

[ImaginAsian]

Try what FBT says and also try a 4 pin molex to see if it still hums. If it doesn't, it may be the Mobo as you suspect. Weird...I've never heard of this one before though.

[Alakazam]

I've got one that does the same thing. I know that the 'soft power' signal, which is called "+5 V Standby" (or "+5VSB", or "5VSB", etc.) is the same output level as the regular +5 V lines from the power supply, but is independent of the other provided voltages and is always on, even when the rest of the power supply is turned off. A small amount of current on this wire is what allows the motherboard to control the power supply. Like you said, 12v should not be affected, I wonder if the PSU is getting ready to take a dump?

[prod]

Ok I put the old voltmeter on the fan connector, Im getting approx +1.5v AC signal with +1v DC offset. Same at the power supply connector when it is plugged in. When I unplug the ATX connector, I get nothing.
Question: Are the outputs of the power supply affected when you unplug it?
If so, then Im not getting an accurate reading. I need to isolate whether the signal is coming from the power supply or the motherboard.
And one other thing, the humming does go away if I cycle the power switch on the back of the power supply, but is not affected by the front power switch.

[James]

What voltage do you get at the fan header,when the system is running?

[floppybootstomp]

The ATX connector from the PSU needs to be plugged into the MB to make it active. There's a short circuit present on the MB between two of the connectors that switch the PSU on.

You can activate the PSU by shorting two of the connectors, I think it's the green and any of the the black (0 volt) wires, but don't try that as I'm not sure!

I asked this question myself a whiles back, I wanted to check all the rails on the PSU without it plugged in the MB, but I've forgotten what colour wire it was.

That voltage present on your fan is mighty odd I gotta say.

[frombadtraverse]

The power supply needs a load on it to stabilize the rails. Most switching supplys need about a 10% load on each leg for stability. Running a switching supply without a load is not recommended.

The AC is curious, could be you have a ground loop with other equipment that is plugged into your computer. Are all your outlets grounded. You might try unplugging other things like speakers, telephone line, etc. to see if there is a grounding problem.