‘When he noticed that it was looking at him. The look frightened him as he tissed at it.’
‘He lay still, his heart throbbing, blowing and tissing and slavering.’
Another otter sound that Williamson uses a number of times and which is not found in the 600,000 word version of the OED. Tissing seems to be what they do when they are happy, sad, scared, or playful. In these examples, Tarka is spooked by the defiant glare of a fearless bird, or is otherwise in a tizz.
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 24 and 119