An ELT Glossary : An Alveolar Flap/Tap

Many speakers of American English use a flap (or tap) consonant rather than a plosive in forming the /t/ phoneme. Flap sounds are similar to plosives, but the articulators meet too briefly to allow a build up of air. In the alveolar flap [ɾ] that serves as the /t/ allophone, the tongue is raised to hit the alveolar ridge very quickly. 

Because this is an allophone (ie never creates a meaning difference) rather than a phoneme, many native speakers do not recognise it as being a distinct sound from the other phonemes of English but perceive it as being a voiced alveolar plosive -  ie /d/. This is a good example of how difficult it can be to perceive differences in sounds, even when used in your own language, when they they are simply allophones rather than phonemes