The moving parts are the fans and the hard drive(s). Determine the baseline noise level by booting in safe mode and not logging in. If the fans seems noisy then, they may be running too fast because of conditions such as a faulty temperature sensor, poor ventilation, or dust buildup -- or the bearings of one may be failing.
Noisy hard drives can be replaced, if necessary.
Noise could also be caused by transmitted vibration in loose parts such as drive sleds.
Use a program like smcFanControl to see what RPM's the fans are turning at. If they are spinning at normal speed, could be your 4870 fan, or one of the hard drives is going south.
I have a 2006 Mac Pro and all 4 fans are turning between 599 and 1062 RPMs. CPU 963 RPms, Exhaust 1062, I/O 963, and Power Supply 599. When I use Handbrake, I adjust some of the fans closer to 1500 RPMs to compensate for 8 cores running at full capacity. Mine is nearly silent right now as well.
Thank you all for the comments -- I have rebooted in safe mode, and I would still say the machine is noisy, although perhaps not as noisy as when I'm using it for something which requires the processors to do some work. It's as though there are a number of fan noises which slowly come on, overlapping each other rather than one thing which is making all the noise on its own.
I will try smc Fan Control later today -- but going back to Linc Davis's response, now that I have established it's not silent in safe mode, what's the best course of action? Should I take the whole thing in to an apple store to have them look at it? I've tried to clean the inside thoroughly, but not to the point of removing the graphics card or hard drives.
I'm so grateful for your help -- thank you for the speedy responses
graphic card and make sure it is clean and firmly reset. It probably has a latch on the side of the slot lane.
disk drives - tight but not too, and some users put in noise dampening washers
memory riser cards (2) and the DIMMs though you had to do that when you added RAM and Risers and DIMMs do require a rather firm push to click into place.
loose fan housing is also possible.
not really a lemon. but maybe a part is loose or a screw missing (lteral and figurative)
as long as it passes the Apple Hardware Test and is stable etc and doesn't kick up gpu or memory errors.
Mine was quiet until I added an older 10K Raptor 74GB that seemed to not be that noisy but caused the case to "humm" and the vibration of it and other parts "clash" or setup some discord.
The way mine got quiet again, was to use some washers and insulation padding where metal on metal was an issue, then, to put it on its side. Quiet again. But there could be a high pitched humming whine that was almost impossible to hear, but very irritating. One of those things hard to detect but could hear even as I walked into my office.
THIS FIXED MY NEARLY IDENTICAL COMPUTER AND SYMPTOMS:
I've been trying to correct the full-speed fan syndrome on my mac pro (2008 2.8 quad core) for months now. I finally downloaded and installed iStat, then saw that the "Northbridge" temperature was way higher than any other temp (it typically ramps up from 140 to 259F).
What fixed it finally was simply disconnecting and reconnecting the northbridge heatsink-to-motherboard connecter (simple two-prong connecter just left of center in this photo: http://forums.macrumors.com/showthread.php?t=544597&page=2).
Perhaps a weak connection there led to erroneous resistances and a false temperature reading.
The connecter is accessible by just taking the side panel of the computer off and removing the video "card".
The Northbridge temperature is now stable around 122F, more in line with the other temps.
It's kind of embarassing what I went through in the way of troubleshooting, but let's just say I wish I'd started with iStat six months ago.
Good shot. I recommend off the bat that people install SmcFancontrol 2.3 and Hardware Monitor and if necessary set HM to alert if any sensor reading exceeds a threshold (but you have to set each manually).
Did you ever touch or go in that area in the past? or was it just a freak event? and got loose by itself.
Hello all of my fellow noisy fan suffers,
After trying many things calling apple support, also the people that sold us the computer, reading many posts about resetting this or installing that I sort of gave up.
Then one morning when the temperature in town was near on 0 celcius and the computer had just been turned on, and the fans where hailing away at full throttle, I put the computer to sleep.... ahhh no more noise. but alas I had work to do so had to wake it back up again.
Funny thing is when I woke it up the fans where not going at full throttle anymore, infact they where as quite as a mouse. This I found interesting and a huge relief.
This has not stopped the problem, most mornings no matter how cold it is fans still play up but a quick shhh my dear machine go to sleep for a minute then start it back up and no more noise.
I am still waiting for the solution (maybe buying a PC "HOPE NOT") Hope more that apple will get off their collective backsides and fix the problem.
But for all of us out there that is having this ear ache of a problem maybe my solution could help just a little.
All the best
P.S. This is the same message I gave to another post but would like to add that you should check dust build up between the fan and and what they are there to cool.
"Good shot. I recommend off the bat that people install SmcFancontrol 2.3 and Hardware Monitor and if necessary set HM to alert if any sensor reading exceeds a threshold (but you have to set each manually).
Did you ever touch or go in that area in the past? or was it just a freak event? and got loose by itself."
I don't think I've ever bumped that connecter. I suspect some form of nearly invisible, possibly bi-metallic corrosion occurs with electrical connecters sometimes and the simple act of unplugging and plugging it back together reconnects the contacts as per the design.
I actually bent one of the prongs on the motherboard very slightly to creat a bit of side force in an attempt to prevent another occurrance.