Im trying to wrap my head around this. Ok, so theres a squeaky bed spring sound but more plasticy. It does not do it at idle, nor is it effected by speed, turning, braking, etc. A bed spring sound is made when you sit on the bed or move and the spring makes its noisy sound. Is this sound constant? Or just when you hit a bump? Whats odd to me is that it only happens when the car is running. So if you had the car parked, and off, and jumped on the bumper or pushed on the hood up and down to make the car bounce - there would be no sound? This would mean to me that its not suspension related. I would check your fan, fan shroud, belt, pulleys, wheel bearings. It could be anything at this point. Hard to tell with the lack of information.