iMac 24 inch made loud snap sound

Question

99% sure that my early 2009 iMac 24 inch has twice made an excruciatingly loud snapping sound.

Details:

• Sound was extremely loud, like a firecracker snap.
• Occurred both times in the late evening, two days apart.
• No electrical storms. Clear weather. Temp 40s/50s.
• RAM was upgraded in Aug-2011 with OWC RAM from macsales.com as described here: (8.0GB(4.0GB+4.0GB Kit) PC8500 DDR3 1066MHz SO-DIMM Memory Upgrade Kit for all MacBook Pro 13", 15", & 17" 2008-2009-2010 'Unibody' models; all MacBook 13" Unibody; Mac mini 2009 & Later; iMac 2009 Models.) Never had a problem with that.
• No other modifications were made to the iMac.

First occurrence (around 11:30PM)

• Was across the house and ran to figure out what it was, it was so loud that I had imagined someone smacked a window with the back of a spoon. Or imagine the sound of someone shooting a BB pellet at a window without it breaking the glass. More of a snap than a pop.
• iMac was plugged directly into wall socket (no power-strip), was in sleep mode.
• Did not suspect iMac or any other device though after inspecting my home, its windows, etc I began to wonder if it was electrical.
• There was no smoke, sparks or odors.
• I made sure there was no heat being called for on the HVAC, at that time.

Second occurrence (around 9:00PM two evenings later)

• Was sitting within 5 feet of the iMac when it happened the second time and 99% sure it was the computer.
• I jumped out of my chair, it was very startling.
• I made sure there was no heat being called for on the HVAC.
• After first occurrence I made sure to plug the iMac into a powerstrip.
• No visible sign of a sparks, no odors.
• Once again I checked outside and the windows, siding, etc in that corner of my house.

After occurrence #2 I unplugged the iMac from the power strip and the power strip from the wall. There has not been another snapping sound since, its been 2 weeks. My guess is that it had to be the iMac.

I havent tried to turn it on yet - I am a bit apprehensive after that noise. I've got two young toddlers in the house all day and also dont wish to risk fire or at the least startling them.

I may ask my local Apple store next time I am there but my guess is that they no longer support this age iMac and if its a power supply or the monitor power board, or similar, then it may not make sense to fix it.

Has anyone else experienced this?

Answer 1

The description sounds like a capacitor blowing up. Common enough occurrence in a hardware lab, but should not normally happen on a production computer. Could be some not-quite-top-of-the-line caps got used in your iMac, and didn't die until just now. If the Mac is running fine, then these were probably "bypass" caps, used to smooth out (minor!) voltage ripples. There are usually dozens of these caps around the major chips, and also often at the edges of power planes. 1 or 2 blowing up won't cause the computer to fail outright, but if more occur you could start to see inconsistent behavior or crashes as the chips react to fluctuating power levels.

And, remember, if these are "bypass" caps, we're usually talking very small fluctuations, like a 2.5V power plane getting ripples +/- 0.5V, so there shouldn't be any safety concerns.