Well the answer as in many things these days is it depends.
Some are very simple and do have a kind of pulsing taken from ripple on the
mains. Others seem to not do this, indeed poking a phototransistor connected
to an amplifier shows many different results. the same seems to go for CFLs
as well.
You would need to know what circuit they were using etc to figure out why.
One particular led in a stood across the road has a 1khz whine when point
the device at it but modulated onto a 100 hz buzz.

I often wonder if there is some jiggery pokery going on to drive leds hard
for split seconds to make them brighter.
Brian

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"William Gothberg" "William wrote in message
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Do switch mode power supplies flicker in time with mains? Specifically
LED power supplies in commercially available domestic lamps. By in time,
I don't mean at the same 50/60Hz, but anchored to it. I.e. if you have
several such lamps each with their own built in supply, will they all
flicker in time, using the mains frequency to keep them in time, or will
they be random, making the room overall not flicker due to them all being
random? And is there any way I can test this? I tried taking photos of
them, but my camera only goes as fast as 1/2000th of a second, which shows
all the lights at the same brightness each time, I suspect the flicker is
above 2000Hz.