In December, 2010, I purchased a Pfaff Quilt Expression 4.0 and after I sew on it maybe 3 to 5 hours, it starts knocking like it is about to throw a rod. Have taken it 3 times to dealer where purchased and their repairman heard it knocking 1 time, but he won't run it long enough to knock. Has anyone who owns this machine had any trouble? Thanks for your help.