I thought only ducks went "quack"

There are a large population of grey squirrels in the area where I live. Up until recently these small furry creatures have been quietly scurrying - swiftly making their way from one patch of green to another and then running up any nearby tree. This week, however, there have been a number of these squirrels that have been up trees and letting out a very loud quacking like sound. Perhaps more like "quark."

As it is now Autumn - it cannot be a mating call. The last squirrel that was making this unusual call was on Tuesday and it sat up in a tree, looking at a ringed dove and appeared to be calling to the dove in this very loud quacking or quarking type voice.