Timing chain cover

I had a mechanic put a new timing chain kit on my vehicle. It was misfiring and had no power. When he tore it down the tension-er had broke. He put a new timing kit on it. I went and picked it up and heard a noise when it idles up to about 2000 rpm's. I took it back to him and he said the chain has rubbed a hole in my timing cover. What could be the cause of this? Any help would be greatly appreciated.