I have had some terrible noice from my HK speakers a couple of times as well.
It sounded almost like a blown speaker, but it is not from the same speaker every time so I suspect the amp to be faulty.
Dealership can`t find anything as the error only occures every now and then.
I am interested in knowing if this turns out to be a common fault as my dealer states "they have never heard about it before".
2013 F21 116d Msport
2007 E91 325d M sport
x 2008 520dAT, x 2006 320d M sport, x 2004 320 dA M sport, x 2002 Mini Cooper S