I've seen a grid pattern in drum scans from a professional lab and was told that it's normal and that I was getting the best quality possible. I attributed it to the fact that the lab scanned at 4800 ppi with a scanner that had an optical resolution of 4000 ppi; in other words, an interpolation problem. It could have been an electrical interference problem but that isn't really a very good excuse for a "high quality" lab. With my own drum scanner (same model) I have never had this problem although I only use optical resolution and have it plugged in through a voltage regulator and RF filter.