Its probably just from leftover coolant from your flush in May. One more flush should get it totally cleaned. If you really want to be sure, drain the system and refill with pure water. Drive for a day, then drain and refill with pure water, drive a day, etc. Repeat as many times as you wish, but the clean water getting put in will pick up the gunk as it flows, and it will come out with the drainage. You get the point. You can also buy flush from NAPA, but that won't be necessary, really. Finally, refill with 50/50, make sure your new coolant can stand temps as low as New England can bring.
