BIRD-SOUNDS IN MY WEST YORKSHIRE GARDEN

I have a lot of different types of birds visiting my garden, some visit regularly, others come and go.  Mostly I see Blackbirds, Magpies, Collared Doves, Wood Pigeons, Blue Tits, Greenfinches, Goldfinches, Coal Tits, Robins, Great Tits, Sparrows, Chaffinches, and Thrushes.  I have absolutely no idea what bird(s) is/are making this sound but I've heard it on and off for a few days now.  I am wondering if there is a nest high up in the trees, out of sight, and I am hearing parents and perhaps chicks.  It's a chattering, squawking sound rather than a whistle and reminds me of budgerigars.  The first time I heard it, I thought it was a squirrel raising an alarm. 

I have absolutely no idea what bird(s) is/are making this sound but I've heard it on and off for a few days now.  I am wondering if there is a nest high up in the trees, out of sight, and I am hearing parents and perhaps chicks.  It's a chattering, squawking sound rather than a whistle and reminds me of budgerigars.  The first time I heard it, I thought it was a squirrel raising an alarm.