Are you assuming that the strange sound is being emitted by the pair in the cedar tree, or do you know that? It's conceivable that you have two new birds! That said, the mistle thrush alarm call is more like a night jar than the fieldfare alarm call. And if they're really nesting, then mistlethrush is resident, fieldfare is a winter visitor. Surprised you've not seen mistle thrushes before, though. They're not that common, but we have them gobbling our holly berries every year.