Wiktionary
interj. 1 (context nonstandard English) (alternative spelling of uh-huh English) 2 (context nonstandard English) (alternative spelling of yoo-hoo English) 3 Expression of mild surprise or disappointment.
Wikipedia
Usage examples of "uhu".
Not only have I seen his rejections, but Abbas Effendi imitated his voice unmistakably, a deep, strong uhu, uhu, calculated to strike terror into mammals as large as a gazelle, and birds the size of a bustard.
In the night it goes uhu, uhu, and every time some poor unfortunate bugger cops it somewhere or other.
A frozen silence inside the tent and a startled silence outside followed the last Uhu, and in this silence they heard a still larger voice cry 'Strike that tent up forward there.
A deep strong voice from the dunes came almost immediately after these words: Uhu, Uhu.