Something's turd in the garden......come on Sherlock...help me solve the mystery!

Something stirred in the garden...at the end of it to be precise. For quite a while now...at least the last couple of months... nights have been disturbed by a raucous, rhythmic scream. Not a fox. Or a muntjac. Or any owl call I know. And then this morning the mystery creature had left its calling card near the shed. Oh my goodness! I would not like to meet the owner of the sphincter that generated a stool of such epic proportions...but it has spurred me on to find out what it is. Tonight, intriguingly, it called its distinctive trisyllabic call just below the conservatory window as I sat only a few feet away but I could not see it despite its proximity and staring intently in to the gloom. At some point (ie when I own one) I will set a camera trap and catch the creature on film but for now the creature's identity remains a mystery...