It's a rare but known issue that can happen with a particular combination of drives running at slightly different rpm's. A difference as low as 10rpm is enough to cause the throbbing noise. But like I said it shouldn't do any harm to the drives. It can however harm your sanity.
If the noise is bothering you, you can suspend the drives as I said above, it's almost guaranteed to end the noise and also vastly reduce the seek noises. You can also unplug one drive at a time to find the culprit, then swap it with one of the the drives in the other cases. It could make the other case start throbbing too though.