Get yourself an "
Automotive Mechanic Stethoscope"
You lay down the end of the rod on parts without applying pressure to find where the noise comes from.
It's designed for finding higher frequency noise like rattling bearings.
you can tape or tie-wrap a smaller needle or pin to the stock tip if you need to.
Also, check
capacitors. they are often the culprit.
We already tried a medical stethoscope but you hear: nothing. Trying to couple the noise mechanically with a "rod" is indeed an interesting idea. And can probably be simulated with a q-tip and the medical stethoscope...
What we also had tried a while ago is to use a tiny detached 4mm electret mic from the GTA04 (which makes it possible to listen to people speaking in 10m distance). But it was still not sensitive enough to clearly identify the source.
Yes, it could be the capacitors (electrostatic speaker). We have ceramic capacitors (to avoid tantalum and ageing) but they may make this noise...
It could even be the PCB: if wires with high current are too close to each other - and we have only 100µm isolation on the prototypes and pulse currents of several amperes - the electrical and magnetically fields could squeeze and stretch the PCB material.
Why is the noise USB related? There is a DC/DC converter for making 5V USB-Host voltage from the battery voltage. And this DC/DC has a converter frequency in audible range. How much energy is shifted around depends on the power consumption of the connected USB device. And an active memory stick draws more than an inactive. So USB operations change the power demand and pattern of current pulses. And some component (inductor, capacitor) in the power supply chain converts these current pulses into acoustic waves...
Regarding Wifi+Antennae: we want/have to change the ceramic chips antennas and have to find out first which one (out of 3 candidates) is the best one. It was difficult to even find some which are small enough, easy to buy, cheap to buy and are not too critical how they are mounted (goal: reproducible mass production). But now we have selected 3 models waiting for being measured and tested.
Why does this require a PCB layout modification? The prototypes up to now (Mainboard V5.1.3) have no dual-band antennas. When we switched from a WiLink6 module to the WL1837MOD a while ago we overlooked that it can and should handle two dual-band antennas for MIMO diversity (improving WiFi significantly, especially for small devices). But the current PCBs are designed for one 2.4 GHz and one 5 GHz antenna only. Placing a dual-band antenna there is not possible because a drop-in replacement with same dimensions simply does not exist.
And one more thing did turn out during evaluation: Chinese manufacturers exactly do what you tell them without asking. Because we had a bug in the layer setup data of the EAGLE board file, they simply produced as specified there. We didn't notice that bug with a local quick turnaround manufacturer because they ignore what the EAGLE board file specifies and they used their standard-PCB stack. So the early prototypes where antenna matching was calculated and basically tested were different from those which went to prototype production.