any foam really... you want to isolate the drive from the bay to reduce vibrations.
By the sound of it however it shoulds as if the drive in question has bad bearings. If the drive has allways done it its probably not serious, but if its just started recently it may be time to get the unit replaced.
Drive bearing can keep on going for years... or they can just get really bad & fail suddenly.
<b><i>"Why don't you try practicing random acts of intelligence and senseless acts of self-control?"</b></i>