What nocturnal animal sound am I hearing in the trees in Maine?
My initial guess (knowing almost no information) is that it's a Northern saw-whet owl (Aegolius acadicus). These owls live year round in Maine but also increase in number as some migrate from Canada through Maine for winter. You can hear their advertising song here.

Northern Saw-whet Owls have a distinctive too-too-too call -- an insistent series of whistled notes on roughly the same pitch, given at a rate of about 2 notes per second. Males calling to advertise their territory can be heard up to half a mile away. Other males respond with a softer, faster, lower version of the call.