Hello , since i upgraded my 2012 mac mini to El capitan i get strange sounds of my thunderbolt display.For example if i delete a file i get a light crack then the delete sound and after this around 30 sec of an hissing noise out of my thunderbolt dispay speakers.If i change the sound output to my Edifier 730D then this cracking/hissing noise doesnt happen.The same hissing noise remains for about 30-33 sec after videoplayback in a safari tab.The same happens if i connect my macbook ret mid 2014 to the thunderbolt display. On an external SSD i still have a copy of my last yosemite install and if i boot from it none of the above things are happening.What i've done till now is SMC reset , nvram reset , and today a clean install of 10.11.2 , the problem still persists. Is this a known problem? thank you very much for your help and excuse my english not a native speaker. Best regards balooo ps. i posted the same text on the official apple discussion forums.