Both Clifford and Doogie do that but only slightly different and not that loud. Clifford was raised as a baby along side a real human baby, and she would say "ummmm" to him on giving him his Zupreem Handfeeding Formula just like a mother spoon feeding her baby to get them to eat things they may not like. Doogie's "ummm?" is upswept like someone asking a question and he has even taught this back to Clifford. Clifford does the upswept one usually in response to getting a McDonalds ice cream cone