Rattling sound coming from Brake booster and Master Cylinder
 

Ok, so here goes.....

Earlier in the week I posted a thread about a knocking/rattling sound coming from the engine bay of my 2014 FRS Automatic when at a full stop with the brakes applied in drive. Not in Park, Neutral, or reverse. After much searching around to include looking for loose heat shields I could not find anything out of the ordinary.

Long story short I went to Harbor Freight and picked up a stethoscope (my old one broke) and was able to replicate the conditions in my driveway with my friend driving the car and stopping so I could get out and see if I could track down the sound. My first suspicion was the secondary vacuum pump but it sounded normal. On a whim I moved over to the master cylinder and with the stethoscope probe plain as day I was able to hear the rattling sound coming from the master cylinder and to a lesser extent the brake booster. Also, pumping the brakes hard causes the same symptom but with a dip in RPM's that recovers quite quickly. Also I would add that the brakes feel perfectly fine with no symptoms other than the sound.

Any Idea what could be causing a knocking/rattling/high pitched squeaking sound from a master cylinder or brake booster? A bad check valve? A vacuum leak? Or bad cylinder or booster?

I'm stumped. I'm going to investigate further but wouldn't mind some opinions.