posting a picture would help us know its age and help you tell you what to do.
Are its eyes open? how big is it?
Are you 100% positive it's a baby rat?
How did you hear a heart murmur? Where is the sound coming from? are you sure it's not a clicking sound from the breathing?
It's hard to hear a heart murmur in humans, so for rats...
A heart murmur is usually fine anyway, not much you can do there.
If the sound is coming from the breathing, the little critter may have a respiratory infection and will need antibiotics.
You should try to get it to a vet! Or a wildlife rehabilitator if it is a wild one.
What color is it?