Reducing rattling PWM noise of a fan

[AndreyS]

Hi there, I'm having a lab power supply with very annoying fan noise. First I thought its noise comes from dislodged axis or lubrication dried out, but in fact the noise comes from coils. Here video demonstration: https://streamable.com/q4cqt7
Googling the issue has led me to this articles:
1) https://ww1.microchip.com/downloads/en/appnotes/00771a.pdf
2) https://jgaurorawiki.com/a5/remove-fan-pwm-noise

To sum up, the solution would be to solder ceramic capacitor across base-emitter of the transistor. In my case there are two transistors in darlington configuration. To which transistor should I solder a cap? I'm quite confused.
Or maybe you've got a better idea how to deal with this?

[benj38]

I have not looked at the articles you included, nor am I commenting on the effectiveness of the proposed solution of adding a capacitor.
My comments are of general nature:

 1. A Darlington pair is usually treated as a single transistor, as far as the rest of the circuit is concerned;  you should consider its emitter to be the emitter of the transistor that is not connected to the base of the other transistor. The collectors of the two transistors are tied together and make up the Darlington's collector.
 
2. Why don't you just try and see what works? it takes less than 20 seconds to de-solder the capacitor and solder it again to another location in the circuit, so just experiment! You can hardly damage anything by testing these different locations.

[AndreyS]

i should have clarified some things:
15V 0 15V ac goes into bridge rectifier than into 7815 and 7915
fan (24V) is driven by ne556n (dual bipolar timers).
The thing with experimenting, I just must not mess up. Thus seeking some guidance from you eevblog forum users

[Gyro]

If it helps, I fixed a similar fan PWM rattle on my GW Instek GPS-2303 PSU using a fairly basic RC filter...

https://www.eevblog.com/forum/repair/cured-my-gps2032-psu-rattle/

[fmashockie]

Is this device under test something you are building or an existing device that recently started exhibiting this symptom?  If the latter, I don't think you've ruled out a bad fan as the cause.  The second article you shared even mentions that filtering the PWM would not fix the issue if it is caused by a bad fan.  So again, if it is the latter, I would try replacing the fan first.