Hi all I'm new to this site and hope that someone out there will be able to help. last week the car sounded as if the exhaust was blowing and then it started running really rough plugged the diagnostic machine and came up with P0300 multiple misfire changed the coil pack and it started and ran fine but it still sounds as if it is blowing somewhere so took it to an exhaust place and they said it's not blowing,but it sounds awful I think the manifold is cracked is this possible and are they prone to cracking any help would be great full cheers.