This sounds just like Audrey...she's just been diagnosed with advanced heart disease which was detected by a slight heart murmur - a swishing noise over her heartbeat.
She's been goven around 3 months as its so advanced. At the mo she's on 4 lots of meds and is now refusing one so I dont know what will happen now. I really would get it checked out as cats can die very painfully from symptoms surrounding heart disease such as blood clots.
Our cardiologist told us it is not uncommon with outdoor cats with murmurs that they go off somewhere and dont come home because they have suffered a blood clot which paralyses their back legs.
Its just so awful.
If Sweep is insured then he will be covered to be referred to a cadiologist - mine has claimed directly with Petplan - thank goodness!