Last checkup my cat had, the vet said she heard a slight heart murmur. I dont remember exactly what she said, but i think she described it as a slight extra sound. Is there anything I should look for to indicate if anything bad is happening? He's about 3.5-4.5 years old (got him from a shelter, dont know), very active and happy. Ive heard that this can be a breed problem with the maine coons. With this sort of thing, is there going to be any early indication that its bothering him or getting worse, or will he just die suddenly one day?