What is your idle speed. FWIW, my 87 930 had the same symptoms. When cold, everything was great, however, when it got hot, it had a dull knock as you described. I took it to my mechanic and he told me that if the idle is below spec on some turbos, the intermediate shaft will sometimes make noise. In fact, my idle when hot was around 750 rpm. As he raised to idle up to around 900 or 950 rpm, the sound completely went away. No more issues.
2016 Porsche GTS Club Coupe (60th Anniversary)
2007 Porsche GT3; 2014 Porsche Cayman S
1981 Porsche 911 SC
2010 Audi Q7 TDI; 2002 Audi A6 4.2