Sounds like over-crowdedness in the hive. Often, outside temps can be cool, but over-crowded hives can exceed 95 degrees (the ideal temperature for egg hatching and brood rearing) and to keep the temps at or near this temp, the excess bees need to beard outside the box.
This will trigger swarming almost everytime - if you have bearding in cooler weather, so expect a swarm soon. The only way to avoid it (if at all) is to add a hive body (super) with drawn comb (ideally) or frames with foundation (generally) to give the bearding bees somewhere to go and MORE IMPORTANTLY something to do!
You could search and kill off any queens or queen cells, but letting a hive swarm isn't the worse case scenario, your hive could benefit by swarming. Of course the bees that swarm MAY NOT have the best of times finding a viable home that they can prepare for Wintering, but the bees left behind will have "RELATIVELY" a lot of honey stores to go into the Winter with and that is always a good thing.
Many people would argue (and rightly so) that late season swarming should be prevented, but realistically to the backyard beekeeper it comes so quickly that before you get a chance to stop the swarm, the swarm occurs.
Personally I don't mind letting the swarming happen. Often you can reclaim the swarms, but just as often they find their way back into nature and help repopulate the feral bee count. The real problem which occurs is COMPETITION for food sources for your hived bees. Swarms who are luck enough to find a home that is functional as a hive are very accelerated as nectar and pollen collectors, compared to a hive which already has a hive at (or near) capacity. So evaluating your area (as far as food sources go) is important whether you want to allow or prevent swarming, or letting bees return to the wilds. The other issue of course is infestation of varroa and the inability of a feral hive to protect itself and later pass on the varroa to your colonies.
There are pluses and minuses in everything, but your hive is overpopulated, at least it sure sounds like it - adding space and removing any swarm queen cells is the only realistic way to prevent swarming - you might need though to concidar whether you want to build the hive up (late in the season in MOST of the US) or let nature take its course and THEN move the hive once the population is more balanced to the available space.
Good luck and as always, look for many opinions, I'm just another beekeeper, Beemaster is just a name, not a title