E350owner

My 2014 E350 (21k miles) just started making a weird Uh Oh high pitched sound when depressing the brake pedal. It come and goes. When it is making the noise, I can pump the brakes and the sounds does not go way for several pumps. There is no sound of squeaking brake pads outside the car. The noise is similar to that of a small dog making a chirpy squeak sound like an Uh Oh. I've noticed it happens more often when the heat is on or the interior of the car is hot. It is definitely linked to pressing the brake pedal and sounds hydraulic to me. Thoughts? Dealer thinks I am nuts.