hi have a 97 chevy it has the 350 vortec when the motor is cool it rattels until it picks up oil pressure which dont take long about a second. it hold good oil pressure. i just changed the oil in it and it still didnt help it.i can start it up and after it pickes up the oil psi i can hold the gas to the floor and no noise.

 

Like an exhaust rattle? Like the manifolds maybe?

 

no like a main bearing rattleing but it quits when it gets oil psi