Noticing a large number of swooshes on the lower freeband frequencies - roughly 26.500 MHz to 26.900 MHz, with some in-band as well. All 40 CB channels are busy at 1613 UTC.
Some possible speculation for the prevalence of ISM fish hook swishers below the 27 MHz ISM and CB bands:
1. Natural downward frequency drift of the ISM work head resonant L-C tank when in use.
2. Initial calibration at 27.12 MHz is done with the heating work head in free air, rather than in contact with the dielectric work piece, which increases capacitance and causes resonant frequency to shift lower.
3. Factory calibrators may intentionally tune each ISM device toward the low end of the ISM band ~26.96 MHz (rather than 27.12 MHz center) to avoid the wrath of:
_ a. Amateur radio 28 MHz fox hunt vigilantes, frequency cops, or
intruder monitors.
_ b. CB radio operators who might get angry.
4. After the ISM device is purchased and goes into service, it is probably
never frequency calibrated again because nobody cares.