These Microsoft Technet notes on Windows XP Performance might be useful reading.
Resume from Hibernation section,
During hibernation, all devices are turned off, and the system's physical memory is written to disk in the system hibernation file. Before hibernating, Windows XP writes the important parts of memory to the hibernation file in a compressed format.
The speed with which hibernation can be completed has been improved by optimizations in the compression algorithms, and by overlapping compression with DMA (Direct Memory Access) transfers to disk. As a result, the compression time is almost completely overlapped with the I/O for most hardware. The speed of resuming from hibernation also benefits from improvements in the boot loader (which also affects the system boot), and from improvements in device initialization routines that are common with resume from standby.
This is the important part that may explain your hard drive churning,
The time that it takes to resume from hibernate can vary considerably. The amount of system resources needed to resume from hibernation is comparable to the amount needed to boot the computer; but in this case, the computer must also read back and decompress whatever "dirty" pages were saved while going into hibernation. Thus the time to resume from hibernation depends on how much RAM the computer has installed, the applications that were running, and what state those applications were in when the computer went into hibernation.
As a potential solution I suggest you try PageDefrag from Mark's SysInternals suite
PageDefrag uses advanced techniques to provide you what commercial defragmenters cannot: the ability for you to see how fragmented your paging files and Registry hives are, and to defragment them. In addition, it defragments event log files and Windows 2000/XP hibernation files (where system memory is saved when you hibernate a laptop).
Mark Russinovich also has a nice blog to check out for more concepts.
Have you done checks with a fewer number of open applications at hibernation?
Does that change the performance (like few small apps implies better resume from hibernation)?
If you suspect your hard disk life is in trouble, you could look at HDTune.