Hi Tash,
I also bought a golden retriever puppy from a breeder in Denison (via the trading post) 2 years ago. They also trained horses. I'm not sure if it's the same breeder or not but my puppy has also been diagnosed with a small heart murmur. The vet has said that it isn't anything to worry about atm and it's quite common. I'm very concerned reading this. Did the problem gradually get worse over time?