Sail,
I am not sure if a heart murmur will DQ you, but my guess is yes.
Brooks will give you an ECG test and if any abnormalities come up, then you get an ultrasound test. If you fail this then they are probably going to recommend a DQ. Here's the thing, the folks at Brooks are not there to kill careers, they are there to help get you through the initial screening.
My advice is to go there, and if they find a murmur so be it. If they don't find it, then you're good. There's no need to divulge any info unless it is documented in your medical history.
Goodluck,
BB