Hi Scotty, I have a 1986 Porsche 928 V8 with 180K miles with the original mechanical smog pump and if the car sits for more than a week, the smog pump makes a screeching noise at startup but then the noise goes away after 5 minutes. I'm using 20w50 oil in San Diego CA and my belts are all good with the correct tension. Is it the bearing or the pump and why does the noise stop after 5 minutes.

heck, try to get a new bearing set and try that, those things cost a ton, it's one or the other