A boom sound is made by which animal?

The Bittern is an elusive bird seen occasionally skulking through reedbeds looking for fish or flying over a reedbed on its broad, rounded, bowed wings.

The distinctive booms of territorial males can be heard from as early as January at some sites, most typically following mild and wet winters, and can still be heard into June and rarely July. The best time to listen out for booming males is from the middle of March to the middle of May. A booming bittern can be very distinctive at close quarters, but at a distance can be mistaken for a mooing cow or even a foghorn! Often male bitterns give a grunting call before their booming is fully developed, this grunting can be hard to hear unless you are close to the bird and it can sound very unlike full booms.