Hard to know without actually hearing it. However, Blackbirds do often make a soft clucking call as opposed to a full blown alarm call. Anything like this for example?
Play this audio clip
maudoc, XC364599. Accessible at www.xeno-canto.org/364599.
If not can you get a recording?
Hi Tony, Definitely not a Blackbird, we have 3 nesting pairs in the garden and they've all produced & hatched 2 clutches of eggs. This was a very definite 'chuck', it was intermittent rather than repeated and not an alarm/distress call.
Robert