Quick Western Digital reference for internal hard drives
A standard Western Digital model number: WDCXXXXYZ

WDC= Western Digital Corporation
XXXX= unformatted size (10=1GB where 1GB=1,000,000,000 bytes).
Y= features.
Z= interface.

Valid options for features:
B= 2MB cache, 7200RPM, fluid dynamic bearing.
E= 2MB cache, 5400RPM, ball bearing.
G= 8MB cache, 10000RPM, ball bearing.
J= 8MB cache, 7200RPM, ball bearing.
P= 8MB cache, 7200RPM, fluid dynamic bearing.
S= 8MB cache, 7200RPM, fluid dynamic bearing, hot pluggable, long life (server/datacenter drives).
The fluid dynamic bearing drives are also sold under the name "Quiet Drive technology," because they are.

Valid options for interface:
B= Parallel ATA100
D= Serial ATA150

So, to answer your question, the JD drives have the ball bearing while the PD drives have the fluid dynamic bearing. The fluid dynamic bearing is really, really quiet (I sometimes have to put my ear against it to find out if it's spinning), while the ball bearing is typically fairly quiet though some drives are quieter than others.

My system has a pair of WDC1600JB's in RAID-0 configuration and a WC2500PB to back up the array. The WDC1600JB's are noticably louder, but the whole thing plus case fans and processor fans is still quieter than the single fan in my Power Macintosh 6x00 (that I just replaced with a quieter fan) if that helps establish a reference.

-drasnor