06 Nissan Altima rattling while pressing the break.

the rattle could be a shroud covering the exhaust or catalytic , the code has to be read and determined what the problem is , The sales clerk at auto zoo is not a technician , what they sell you may not solve the problem ...

I agree, the rattling is probably a loose heat shield on the exhaust system. Probably for the catalytic converter. Have the computer scanned for trouble codes at your local auto parts store. Most stores will do it for free. If you get a catalytic converter code or downstream oxygen sensor code have the exhaust system inspected before replacing any parts. When vehicles sit, especially outside, they pick up moisture and things on the underside of the vehicle, like exhaust systems, rust and come apart when you start driving the vehicle again. An exhaust system leak can pull fresh outside air into the exhaust stream coming from the engine and trigger a false catalytic converter code, or oxygen sensor code depending on where the leak is in the exhaust system. Hope that helps! Jim