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A schematic of an early version of the unit can be found at the FCC’s Office of Engineering Technology web site. Begin at https://fjallfoss.fcc.gov/oetcf/eas/reports/GenericSearch.cfm then search on the grantee (BYG) and product code (006) as shown on the product label. The original schematic will be found in within the table of exhibit details for the original application.
There are two different frequencies and two different causes for what you are hearing:
First, the phase detector (within processor IC101 and part of the phase locked loop) produces a small but audible 300 Hz ripple. This is not the 25Khz at which the phase detector operates, but rather an internal hardware or software refresh rate. The ripple can be reduced by bridging a larger capacitor across existing .047uF capacitor C110 in the loop lowpass filter (between Q102 and Q103.) On my unit, bridging 0.47uF worked well for the purpose while leaving some measured phase margin for stability. Your results may vary; if you hear instability (anything from a very low frequency “thumping” to a hum,) you must bridge a smaller value and tolerate more whine.
Second, DC-DC converter IC7, which develops the negative side of a split supply for audio processing and the output buffer, uses a charge pump switched at 5KHz. This puts a 5KHz load on the switched, 2 volt positive rail and introduces an audible ripple into the audio chain. If you have “adjusted” your unit for more transmit range as recommended elsewhere, you have increased the load on this converter and hence the amount of ripple it creates. This ripple can be reduced with a 1000uF capacitor in shunt with C35, effectively doubling its value.
Note: Your results may vary and of course, any modification, albeit a harmless one, voids the manufacturer’s warranty and the FCC certification.
