That sounds to me like the so called "rain call" of a Chaffinch. Here's a recording of a similar sounding bird
Click here to play this audio clip
Frank Lambert, XC414236. Accessible at www.xeno-canto.org/414236.
Sometimes they can keep it up for hours.
That was the one! It had a chaffinch shaped head as well - I think I questioned it because it was SO loud, but it was definitely the same call. Thanks so much Tony