Drove car a few times recently for approx 30 minutes over a 25 mile distance, before current freezing weather conditions, left parked outside for approx 1 hour. on start up, at idle there is now a horrendous rattle noise evident from exhaust tailpipe coupled with a vibration felt through pedals, steering wheel etc, the noise smoothes out when lightthrottle is applied but returns when back to an idle state. I suspect exhaust manifolds. Any ideas ?