Latitude E6420 (i5-2520M, BIOS rev A15) - CPU whining, feature allowing to limit c-state depth locked in BIOS

Hello,
my post is related to well-known issue with high-pitched sound produced by CPU or motherboard when CPU is in c-state deeper than C1. Unfortunately I have particullary sensitive ear and I can hear the noise even on frequencies as high as 15-20kHz. The sound is really annoying even though within "acoustic specifications" as stated by Dell.

http://support.dell.com/support/topics/global.aspx/support/kcs/document?c=us&cs=19&l=en&s=dhs&docid=DSN_0A7D5CD2E17F5125E0401E0A55176204&isLegacy=true

Solution mentioned in article linked above (activate Bluetooth) does not apply to my situation as my E6420 is not equipped with bluetooth module.

In my previous notebook Latitude E6410 (i5-560M) it was possible to change value of MSR registry 0x00E2 with use of special utility to effectively limit the maximum depth of C-State. This solution worked.

In my current notebook Latitude E6420 (i5-2520M, BIOS A15) it is not possible to change value of MSR registry 0x00E2 as this option is locked in BIOS (MSR registry 0x00E2 bit 15 is set).

I consider this a serious flaw. Please could you provide BIOS update which would allow manipulation with C-State limits? (i.e. with MSR registry 0x00E2 bit 15 unset). I really do not want to go through BIOS modding and I believe that here must be an easier way.