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It looks like a burst of high frequency occurring when the blue trace is about to switch. Somehow this is being picked-up on the yellow trace - suggesting a problem with the integrity of the ground - a very likely problem if you're using bread-board. The 555 chips were notorious for crowbarring the supply as they switched - some manufacturers were worse than others. Ensure that each chip has good decoupling close as possible to the chip. Also, if the voltage control pin (pin 5? I think) is not being used try decoupling to ground with 100nF.