I have a couple of devices with electret condenser mics, one is an old Casio PDA, another is a 1/8" headphone adapter with a built-in microphone (for iPhone/iPad). Both devices have working mics, but recordings are noisy. The noise is slightly different in each case. With the PDA, recordings have a rumbling noise over them. With the iPhone adapter, recordings have a popping sound that appears every second or so. Does anyone recognize these symptoms and can they be fixed with a new electret mic element, or by some simple modification such as wiring a resistor or capacitor in? (I have a few electret mics taken out of old radios/etc, that may still be good.)