A few options I've used over the years:
Current - 7200 rpm drives reckoned pretty quiet to start with (Samsung F3 and a WD) in Grow Up Japan (how's that for a strange company name?) HDD silencer boxes. These are attached loosely (loose bolts one side, padding to support on the other) into 5.25" drive bays at the front/top of the tower, with an empty drive slot between them for ventilation (the case fan is at the rear blowing air forwards into the case).
This is very quiet, but not cheap and took a lot of expermenting and temperature monitoring to get the case airflow right. Drive temperatures are a little high, but nothing alarming.
Overall, the drives are audible but not to the extent it interferes with mics at all. The whole PC measures around 27dBa @1 metre unless the video card is stressed (it isn't when using Sonar)..
One thing to note - if these kind of drive cases are fitted tightly into a bay you'll get much less drive motor noise than if the drive was bare, but the clicks from seeking won't be silenced much.
Until fairly recently I'd used Seagate Barracudas for years, but they seem to have got louder and less reliable as newer models come along.
Earlier setups I've used are drives suspended by silicone string in a 3.5" bay, with the drives vertical. Not as quiet as above, but pretty effective. It's fiddly to get the drives suspended securely and they tend to move/bounce around if you move the case though.
Drives lying on the bottom of the case on a layer of foam. About as effective as suspending (as both isolate the drive from the case chassis, so noise isn't transmitted into the case from the drive), but the drives can get a bit warm.