Here's the entry from the Concise BWP, Graham
Voice
Advertising-call of ♂ a slow, evenly-spaced series of long-drawn, low cooing ‘oo’ or ‘hu’ sounds, quiet but penetrating, and audible up to 1 km or more. Calls delivered at intervals of c. 2.5 s, the first few in series usually of lower pitch and volume than the rest. Commonest call of ♀ a nasal buzzing sound; difficult to render and variable between individuals, but aptly likened to sound produced by blowing through paper-covered comb. Given almost exclusively when perched—typically from nest, serving to attract ♂; often then alternating with advertising-call of distant or approaching ♂. Contact-alarm call a descending, long-drawn harsh wheezing sound. Full repertoire complex; includes a variety of other mewing, twittering, yelping, and barking calls.