Good Day! I Have A 4 1/2 Yr Old Crown.

Good Day! I have a 4 1/2 yr old crown. Every time, I floss behind it there is a foul rotten odor. No matter how much I floss or brush. My dentist said everything seems fine with it per x-rays and visual examinations. Even the endodontist said the same thing other than there being an accessory nerve left. He took an ultra-sound of the tooth and saw no signs of infection. The tooth does hurt occasionally and has sensitivity to cold and hot. What could be going on with it? Please advise. Thanks! Ms. Hardy