Water Rails are a fairly common up and down the Tees, but being secretive can often be heard and not seen. Their call has been likened to a piglet squealing.
The adult is 23-28 cm long, and like other rails, has a body that is flattened laterally to allow it easier passage through the reeds beds it inhabits. It mainly brown on top and blue/grey on the bottom, with black barring on the flanks, it has a short tail and a long reddish bill.
You may be lucky enough to see one, good luck.