Hi, occasionally there is a fan-like hum from within the tower. It is intermittent but when it appears it hums for 5 seconds, quits for 5 seconds, hums for 5 seconds, quits for 5 seconds etc. If I restart I don't have the problem. It may show up later again because it is intermittent. Is anyone has having this? I hope it is not a normal behavior for this machine? The hum is quiet soft but it is somewhat bothersome to me. PS: Sometimes it never occurs.