i've noticed that when im cruising at around 50 and i want to go a little bit faster my car starts to rattle...i dunno what i could be it usually stops after it switches gears (auto) i haven't changed the tranny fluid in i don't know how long...could it be that i need to change the tranny fluid?

 

You are probably just lugging the motor a little. Do you have any mods? Longtubes could be rattling on the floor or crossmember?

 

just basic mods nothing out of the ordinary...headers, intake, hypertech chip. and its not a rattle excuse me its more like a shake the whole car shakes kinda weird never felt it before

 

When you are cruising at 50 shift the tranny to neutral so the RPM's go as low as possible.
Does the rattle remain?
You should be able to isolate whether its the engine or the drivetrain.

I have noticed on mine that when I cruise around fifty I have a resonance that goes away when I push the clutch in and let off the gas.
This tells me that it is the motor.
I think that the aftermarket exhaust is contributing to this, I have headers and a slightly opened up Borla catback.
The resonance does not feel like its a problem at all, its a slight fast vibration.
As long as your not shaking you should be okay.
The shakes are very bad.