My aunt and uncle used to have a St Bernard/Newfoundland mix, a very beautiful and intelligent dog.
When I was playing any instrument, I wouldn’t get much of a reaction. But at night, when he was all alone in the storage building where he had an old sofa to sleep on, he would sing occasionally.
There is no way one could describe his activity as howling! He would sing his own meldodies, with very distinct separate notes (no sliding involved) of varying length and intervals. Hauntingly beautiful! 