Nope, if you can, remove the drive cage and just put cardboard between the mated surfaces of it and the case to act as a "cushion".

ahh. i realised that when i put my computer on a carpet surface (like my rug ), all is ok..... doh! is that safe? if so, i might just keep it like that.

Not Such a Good Idea.....

IMHO, one of the worst things you can do to is to run electronics on the carpet. In spite of the fact they might only be a 1/4 inch off the surface, the metal bottom plate greatly assists with the cooling. This may not be as severe for a computer because of it's small footprint. Still I wouldn't do it.

Is the surface that the computer is on sufficiently sturdy?

Are one of the fans loose or out of balance?

Some HDD cages have rubber grommets to mount the drives. (notably Antec). If so, are these in place?

Did this occur only when the new drive was installed?
Although it's highly unlikely, the drive could be bad.

Is any of the wiring contacting a fan or the like?

Are you sure you tightened everything correctly?

A couple of thicknesses of electrical tape could prevent a side cover from rattling against the case proper.

When you speak of "humming" I automatically think of the power supply. 60 cycle hum that is.

You've got a screw loose? Well not you, but one lurking in the bottom of the case, maybe behind the whatchacallit.

Try these things for now

Is the surface that the computer is on sufficiently sturdy?
Yes. it was on the bottom of the computer desk (there's a section where you can put your HD)

Are one of the fans loose or out of balance?
No. I checked, cleaned, etc.

Some HDD cages have rubber grommets to mount the drives. (notably Antec). If so, are these in place?
I checked all the HD, made sure the fasteners were tight, that HDs weren't moving, they seem secure.

Did this occur only when the new drive was installed? Although it's highly unlikely, the drive could be bad.
it only occurs when the new drive was installed. but the sound isn't coming directly from the HDD, it's like a vibrating noise everywhere, more so from the power supply.

Is any of the wiring contacting a fan or the like? Nope.

Are you sure you tightened everything correctly? Yes.

A couple of thicknesses of electrical tape could prevent a side cover from rattling against the case proper.

When you speak of "humming" I automatically think of the power supply. 60 cycle hum that is.
I don't know what that is (60 cycle hum). I first thought it was my power supply, but from what Rik was saying, i think the sound is coming from the case.

You've got a screw loose? Well not you, but one lurking in the bottom of the case, maybe behind the whatchacallit.
No, i checked. i feel like pulling my hair out!!!!!!!!

Huuuummmmm.......

60 cycle or "line hum" is the noise you would hear if you tried to plug say a cassette deck or turntable into the input of a receiver which was already turned on. The buzz you might hear when somebody plugs a live mike into the PA. Very low frequency. I hope this explains that, my best shot. Sometimes a PS transformer can vibrate in sympathy to the line AC frequency.

Is it possible that you knocked loose a sound card or something?

Is this drive recognized by BIOS?

You could try booting up the computer with only the new drive attached. To be sure, it won't boot since there is no OS to load, but power will be supplied to all the fans, boards and the HDD. (The computer will stall and give the error message "no bootable device" (or similar)).

A HDD only draws about 20 watts, so I'm having trouble envisioning that the PSU is now overloaded and making noise with only this one new drive added, but I suppose that's possible also.

"You could try booting up the computer with only the new drive attached. To be sure, it won't boot since there is no OS to load, but power will be supplied to all the fans, boards and the HDD. (The computer will stall and give the error message "no bootable device" (or similar))."

can i just turn the computer off afterwards (my new hd doesn't have an os, it's just a media storage) without screwing up the hd?

it should be recognized by BIOS....otherwise it wouldn't work(?)....i don't know. i don't think i knocked anything out of place.

i can't remove the drive cage. it's stuck there. i can remove the HD and put it on the cardboard, but that would cover the breathing holes....wouldn't it?