I was wondering what could possible be the problem when a transmission makes a rattling noise when I am in drive and pressing the brake.  

are you sure it's not pinging coming from the motor???? too much timing??? blocked egr passages...

monolith converter block  rattling do to the support mesh being burned out...
that one tripped me up once... sounded like a rod knock, or the tranny going south...  if you listened carefully it was coming from the cat....  the exhaust pulses were just having their way with it...   like a million mad men with hammers.,..

i tightened it up with a hammer to the  cat converter shell...


undefined noises are hard to find.  

please give us a year, make, model. it'll help us help you. we promise we wont make fun of you, even if it's a KIA.  

LOL. Its no kia, but its close. Its a 94 Toyota Previa, the rattling sound, sounds like it could be my transmission but it also sounds like my muffler is rattling too.  

previas have weird sounds when they run... start at a muffler shop... have them listen... they will probably show you where the noise is coming from... .

and muffler shops wont take it all all apart just to see if it needs rebuilding.. so sorry.. not the problem... but  since the transmission is totally taken apart... we might as well rebuild it.. i quit that job as fast as i could... learned my lesson about selecting who i work for......

wayne....