Although not the same situation and Odie isn't a pup anymore (he's 2.5 yrs), when he was diagnosed with his Addison's Disease in April this year, whilst he was having his meds checked and increased etc, the vet detected a heart murmur (he'd never had one before). It was graded at a 3/4 at the time. Of course I was beside myself but at his next check up about 2 weeks later, it had disappeared! In our case, I think it was down to the quickness of getting lots of meds/being on a drip for a weekend which caused it though.