recently aquired mower having odd vibration issues motor l21zpnc

i have a motor for a steel deck mounted with no modifications to an awesome aluminum deck found in my local lawnboy parts guys junk area. i know that it isnt the blade as it is fine, but it makes a weird rattle at certain rpms when it slows down but not when going up. it's not too loud but concerns me as i cannot find the source of the noise and i am worried it's coming from the engine. i will post a video of the mower running tomorrow as it is too dark to do it now.:frown: pics will be posted soon. just say what you need a pic or video of so i can get this eluding sound fixed.