HDD

First check the machine with the cover off to make sure where the sound is coming from,somtimes the floppy will ingage at start

The drive speed will vary on the system that you have putting in a 7200 drive will not be beneficial if the data bus and other parts are not upgraded to take advantage of the drive,so tho say that the CPU,data bus size and memory will play a part on how fast the drive is accessed

I suspect that the grinding sounds are coming from 1 of your fans, and not the hard drive.

Remove the cover with the PC cold, and hand spin the fan blades.
If the blades wobble a lot, or you feel grit in the bearings, then the fan is the culprit.

If it is the power supply fan, I would consider replacing the entire supply. Since you plan to leave the PC on al the time, you want to have a higher capacity supply to prevent problems.

That said, it is fairly easy to replace either the CPU or the power supply fan.

If it is the power supply fan UNPLUG the power for at least 1 HOUR before you proceed.
Most of these are 12 Volt DC fans. Some even have connectors.
Make sure the new fan is wired and mounted properly.

A good source for replacement fans is Hosfelt Electronics. They sell fans for about 1/3 of what you will pay at Radio Shack.

Chas

As already noted it's likely a fan going bad, but you still might what to check your harddrive manufacturers web site for a utility that tests your drive.
They all have them & it's a good idea to use one regularly (monthly) anyway. You never can tell sometimes when a drive might go bad & the utility might catch a failure & allow you enough time to save any data & have a new drive ready.

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