Maybe not the right answer, but some thoughts...

28636 kHz is a common clock frequency for some types of CPUs. Maybe you're
picking up the clock oscillator from somebody's nearby old Apple II or
something like that.

It can also be used as a NTSC or H-Synch clock for various TVs, VCRs,
DVDs, etc. Do a quick Google of 28.636 Mhz and you'll get tons of hits,
related both to CPU clocks and video synch signals.

Since it is such a widely used clock frequency, that would explain why it
can be heard
by many listeners widely distributed in the US and elsewhere. The various
"tones" may
be introduced onto the oscillator fundamental by characteristics of the
switching power
supply or other such things.

Another possibility might be some new testing of some balloon-launched
experimental telemetry units. Seems some guys in New England did some tests
early last year. Maybe they or others are doing it again. See info on:
http://frodo.bruderhof.com/hambone/04b.htm
Seems if this was the case, though, they'd opt for some more 'conventional'
mode than a
strangely modulated 14-tone signal. Thus, I'd fall back on my first two
ideas.

Ken, WB0OCV