My Brother pc-420 (Project Runway) machine is only one year old. A few weeks ago, it developed a loud knocking sound while stitching at medium to high speeds. I did the maintenance cleaning recommended in the manual (removing the bobbin and cleaning out the lint). I have followed all manual directions for use as well, so I don't think that the problem is due to misuse. Anyway, I took my machine to a local Bernina sewing center that is licensed to service Brother. They did routine maintenance and checked for the problem I described--and thought they had solved the problem. I tried out the machine at home. Now the noise is worse and even present at low stitching speeds! All the stitches are even, and the thread is not catching or getting tangled, so I don't think the tension is a problem. Any ideas? It sounds like the machine is about to come apart "at the seams"--literally and figuratively!!!!