That's a fair question.
In the case of the Cassiopeia PDA (also happening in an old NEC "netbook" type PDA), the rumbling noise happens in isolated places where no air is moving and everything is quiet (ie in an automobile inside the garage [the engine was off, btw], in a walk-in closet w/no vents, etc.) The Casio & NEC are from the late 90s & the noise only began appearing in the past couple of years.
In the case of the iPhone adapter, I have 5 or 6 of these (ie for the car, living room stereo, radio in garage, for work, etc.) I am only getting the popping noise from 3 of the adapters I ordered separately from the first 4. The noise occurs in both my iPhone and iPad, only with the 3 adapters, and does not happen with the other 4 adapters, so I'm pretty sure the problem isn't in the power supply. (I paid less than $1 each & can generally use the ones with bad mics in places where I want to listen to music and don't need the microphone for recording.)
The cover of one adapter popped open and got lost so I cannibalized it to add a headphone jack to below iPhone 1/4" guitar input. That part is working great. However I added a DPDT switch to go between guitar input & condenser, and the condenser is noisy, hence my question.
Also I am generally curious about this since I have 3 devices with electret related noise issues, and it would be nice to fix.
PS here are the devices in case you are curious
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The headphone adapter:
Headset Hands Free 3.5mm Adapter + Microphone for Apple iPod Touch iTouch 1 2 3 iPhone 2 3g 3gS
http://www.amazon.com/Headset-Adapte...482629&sr=8-16
http://www.amazon.com/Headset-Adapte...482655&sr=8-41
http://www.ebay.com/itm/Handsfree-ea...em19c9d0b7f c
http://www.ebay.com/itm/Handsfree-ea...em19c9da22b 5
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The guitar input:
Peterson 3.5mm - 1/4in iPhone/iTouch Adapter Cable White
http://www.amazon.com/Peterson-3-5mm...2&sr=8-1-spell
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The Windows CE devices:
NEC MobilePro 770
http://www.hpcfactor.com/hardware/de...tion.asp?d=129
http://www.necam.com/mobilesolutions...bilePro770.pdf
Casio Cassiopeia E-100 Palm-Size PC
http://www.amazon.com/Casio-Cassiope.../dp/B00000J0EN
http://pdadb.net/index.php?m=specs&i...ssiopeia_e-100