Mac Pro 5,1 here (hex-core) with a Radeon 5870. Several times a day I hear this noise coming from one of its fans. It's a faint, putter-putter-putter noise that rises and fades away usually within the space of 30 seconds. It occurs randomly throughout the day. There are no other symptoms, but I'd rather not leave this to chance or have it be a precursor to a bigger problem, and I want my Mac functioning 100% perfectly. It's hard to tell which fan is causing the sound. I'm going to try using smcFanControl to narrow it down - when the issue occurs I'll set one of the fans on maximum RPM for a second to see if it temporarily stops it. Still it's difficult to determine... has anyone experienced this before and knows what's up? I have a 3 year AppleCare plan, but if I could tell the techs which fan is doing it, that might reduce the time required to have it in the shop.