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"singing" board (audible noise), how to pinpoint?

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roland:
We have a small board (roughly 50mm * 150mm) with medium to complex design (quad core ARM Cortex A17, display, some LEDs, loudspeaker, microphones, networking/WiFi) etc. which is working nicely but there is one big issue: the board produces a clearly noticable, high-pitched whining tone (about 5kHz). After EMC testing of the product, we needed to fit an EMC shield over the CPU and RAM, and this has now unfortunately amplified this noise...

I know the usual suspects, we have checked all inductors and MLCCs near the power supply, DC-DC converters (power companion chip) etc., however we didn't find anything yet that might correspond even remotely to the 5kHz we're seeing (or better, hearing).

Does anyone here have a clue about a method how to pinpoint the issue? Is there any kind of measurement we could do (some special directed microphone or something) that might help to find the root cause of this issue? At the moment we're more or less cluelessly probing around with the scope or running over the board with a mobile phone's microphone (and FFT app). However, since the root case element seems to let the whole board vibrate, it is very difficult to find the element that produces the waveform in the first place. I guess if you had a guitar and didn't know that the string actually produced the sound, you could probe around the body quite some time because the sound seems to come out of the resonant body.

Any idea is appreciated that might help.

Twoflower:
Use a small tube as stethoscope and scan the PCB with it.

ogden:

--- Quote from: Twoflower on February 21, 2020, 05:47:49 pm ---Use a small tube as stethoscope and scan the PCB with it.

--- End quote ---
Use PC with microphone. Hint: old phones came with (wired) headset+mic, you may still have such in scrapbox. Otherwise just buy it for buck or two:

Siwastaja:
Or, use a toothpick or similar to touch/press components one by one, likely a small amount of compression makes the sound stop, helping you pinpoint the suspect.

Bud:
Try using a 1uF ceramic capacitor on a toothpick or on a cotton swab stick as a vibration sensor  , wired to the oscilloscope input to monitor the amplitude.

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