Strange flight call

About 7am a bird I've never heard before flew over our house in Essex and despite spending ages researching I don't have a clue what it could have been.

It was loud, similar volume to a goose flying over, but wasn't at all nasal or honky, more of a rattling repetitive, 'bu-gurrrr' kind of noise, with a lot of reverb on the 'gurrr'.

Does anyone have any ideas what it could have been? It even made my 4 year old jump up and say 'that was a weird bird!' I'm thinking maybe a migrant, but am honestly at a complete loss.