Kdump hangs "Rebuilding /boot/initrd-...kdump.img"

I recently patched several RHEL6 machines. A small subset of them kernel panic with 2.6.32-504.8.1.el6.x86_64. If I go into the boot menu and choose an older kernel (like 2.6.32-431.29.2.el6.x86_64) the machine boots and then seems to hang at:

 /etc/kdump.conf
 Rebuilding /boot/initrd-2.6.32-431.29.2.el6.x86_64kdump.img

Is there any way to investigate this issue? Or does this process normally take a long time (minutes/hours)?

Any help / insight is greatly appreciated.