Andy Wu, I am most certainly not the person to give you a definite answer, but here is what the manual says, and it may help you out:"
Symptoms Quick Check
Noise / Hum / Vibration
• Buzzing noise
• Rattling noise
• Ticking noise
• Squeaking noise
1. Verify display’s USB and Mini DisplayPort connectors are plugged into known-good supported system with up-to-date system software.
2. Tilt display to hinge limits to determine if mechanical noise is generated by the hinge mechanism. Repair/replace mechanism if needed.
3. Play sound sample at loud and soft volume levels to determine if noise is caused by the left/right/ subwoofer speakers or the amplifier circuit.
See Audio Issues.
4. Verify that air intake/outflow vents are not obstructed, inducing a high fan speed.
1. Disconnect AC power cord, remove glass and LCD panel, disconnect left, right, and subwoofer speaker cables from logic board. Then reconnect MagSafe, DisplayPort and USB cables to a powered-on computer. Verify if noise has disappeared.
Yes Audio issue with speakers or amplifier. Go to Audio Issues.
No Go to step 2.
2. Verify that ambient air temp sensor is correctly positioned on fan and securely connected to logic board, and that LCD temp sensor is securely connected to its connector on back of LCD panel, and properly protected by insulating foam.
Yes Go to step 4
No Reseat sensors connectors / readjust sensor position/ foam protection and retest.
3. Determine if noise issue sounds like fan is running unusually fast.
Yes Replace ambient temp sensor cable.
No Go to step 4.
4. Remove fan and inspect blades for damage or obstructions.
Yes Replace fan.
No Go to step 5.
5. Remove fan and rotate blades. Verify that fan blades spin smoothly without interference from fan housing.
Yes Go to step 6.
No Replace fan. Retest.
6. Reinstall fan while carefully ensuring that there are no cables routed under or near fan assembly that might cause interference with the fan blades. After reassembling verify that noise is resolved.
LED Cinema Display (27-inch) Take Apart — Fan Removal
• Glass Panel
• LCD Panel
• ESD wrist strap
• Torx T10 screwdriver
1 Remove T10 screws:
2 Observe cable routing through black plastic guides in rear housing.
3 Disconnect 2 cables from logic board:
• fan cable (4-pin)
• sensor cable (3-pin)
For easier access to reconnect cables, unscrew and flip over logic board.
4 Remove ambient temp sensor cable from fan, preserving black tape for reuse.
Ambient temp sensor just extend exactly 1cm from plastic retaining ring on fan so that it will sit the proper distance from rear housing when installed. Incorrect sensor placement can lead to false temperature readings and unusual fan behavior.
Ensure fan fits securely into foam on rear housing for a snug fit. Air leaks in pressure wall can lead to temperature and/or noise issues.