firstly, how old is the IDE HDD you installed ?
the warranty expiration date printed on the HDD is a good guide
who is the manufacturer of the HDD you added ?
Edit: but realistically it is more likely that a fan (GPU, CPU or another fan) is slightly mis-aligned, possibly you accidentally & inadvertently touched or moved a fan while installing the HDD, it is very easy to do without noticing
remove the lid again, & listen very carefully to see if you can hear where the sound is coming from, if you still think it's the HDD, feel it by touching it because HDDs typically vibrate
and does the HDD spin up at system power up, touch it while you power on