in my other E53 i had a power inverter installed by previous owner (12 VDC to 110 VAC) and it had its own POST (power-on self-test), that included a high pitch sound that lasted for 7 to 15 seconds... drove me nuts until I found the sucker under the passenger seat!!!

are you sure it is not the anti-matter containment field that is interacting with the plasma flow compression control matrix??? It sounds awful similar to the one!!!