Cygnets actually make a peeping sound (non stop) which alerts the mother swan know the location of the cygnets at all times and if they are okay. The sharper or more piercing the peep (almost like a scream), alerts the mother swan of a predator or other danger.

The sound you describe from the male almost sounds between a bleep and snort along with an occasional high pitched bleep. Swans make this sound as a greeting. If he is wagging his tail and bobbing his head at you, he is greeting you and showing his pleasure especially if you or someone else is going to him. The Regal Swan
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