Welcome to the forums keithd.
Blu-ray can have a lot of dynamic range and demanding LFE. I am not familiar with the UP blu-ray though. I would first check your settings in your receiver, particularly your subwoofer pre-out level in your receiver ( I wouldn't have it set past +4). This can be set too high and can issues such as you are experiencing - sending a distorted signal which is causing the flapping/rattling/clapping.
This issue can also be caused by loose screws securing the driver, amp or even having a defective driver like which has been previously mentioned.
Good luck and keep us posted.
