Found this in a Latitude 3540 manual. Could be your C MOS battery
Beep Codes
The computer may emit a series of beeps during start-up if the display cannot show errors or problems. These series of
beeps, called beep codes, identify various problems. The delay between each beep is 300 ms, the delay between each
set of beeps is 3 seconds, and the beep sound lasts 300 ms. After each beep and each set of beeps, the BIOS should
detect if the user presses the power button. If so, BIOS will jump out from looping and execute the normal shutdown
process and power system.
Code
Cause and Troubleshooting Steps
1
BIOS ROM checksum in progress or failure
System board failure, covers BIOS corruption or ROM error
2
No RAM detected
No memory detected
3
Chipset Error (North and South Bridge Chipset, DMA/IMR/ Timer Error) , Time-Of-Day Clock test
failure , Gate A20 failure , Super I/O chip failure , Keyboard controller test failure
System board failure
4
RAM Read/Write failure
Memory failure
5
Real-time clock power fail
CMOS battery failure
6
Video BIOS test failure
Video card failure
7
CPU - cache test failure
Processor failure
8
Display
Display failure