I have a long beep when i start my server - Dell PowerEdge 1800 Server

Look by BIOS and version. It should be noted during initial boot, or even BIOS itself.

you should start by re-seating the RAM first as this is a common problem. if that does not work re-seat the processor. If all that fails remove all add on cards and memory only leaving the processor and see if the beep codes change. Replace the components one-by-one to find the problem piece of hardware. HERE is a guide from Dell to tell you what the beep codes mean and also some troubleshooting tips.

Clean Dust inside the server & update with latest bios ,room temperature should be 18 degree Celsius

It might not be a driver. It might be the monitor. Try another monitor.

I don't understand the beep code you describe, but the 2850 has many beep codes to help understand what is going on.

First ... Identify the beep. Does the beep start as soon as you power it on, or after several seconds? What is the pattern of the beep - is it a repeating set of beeps - 3 sets of beeps between 1 and 4 beeps ... like 1 beep-3 beeps-2 beeps, or 3 beeps, 3 beeps, 1 beep? Or is it a steady repeating beep - about once per second?

Second ... is the LCD panel on the server blue or orange? If orange, what is the error code(s) it is scrolling?

If it is a steady, once-per-second beep, it is probably your RAID controller. Does the system boot to Windows/OS ok? If so, use (or install) OpenManage Server Administrator (Managed Node from Dell's Support Site) to check the Hardware Logs and check the RAID array (under Storage/Virtual Disks). If it is not booting to the OS, then go into CTRL-M to see what the status of your array is.

If your PowerEdge will not boot but only beeps on start, there are a number of things that could be the issue. One thing that can be done is to document the beep pattern and use that to look up the issue or post the beep pattern here.

replace with a new card. The beeps are indicitive 0f failed hardware 2 beep 2 beeps = video card series of 3 beeps keyboard ,malfunction etc etc so whichever card now gives you one long beep you had better start there.....

I would first check that all the fans are turning when you first power it up. It may have fan speed sensors on them, and one or more fans aren't coming up to speed. I would check the CPU fan(s), as they are usually smaller, and tend to go bad before the other larger ones.

Check the ram and VGA card if not integrated.

re-install the old bios. evidently the new flash bios is not compatible with your machine.