Hi wajdi abu lail,
There is a possibility that the fan on the Dell Latitude E6400 might be running but it is not making sound. However, to determine the same request you to follow the below steps:
(1) Turn on the computer.
(2) At the Dell logo, keep tapping the F12 key until one time boot menu appears.
(3) On the boot menu screen, select the Diagnostics option. The PSA+ window is displayed, listing all devices detected in the computer. The diagnostics starts running the tests on all the detected devices.
(4) Once the test is completed, a message prompt will appear “if you want to continue memory test” select no.
(5) Then 32-bit Diagnostics will be initiated, select custom test.
(6) Now select fan as a specific device and run the test.
(7) Once the test is completed the results will appear and make a note of the log displayed.
You may also clean the air vents with a can of compressed air.
Do reply with your findings.