I've seen that screech and it can be fatal!  It happened on a friends '84
and it was the alternator.  The regulator quit and the alternator went to
full unregulated voltage.  It was a terrific load on the engine, hence the
screech.  The bad part was, about 5 minutes later it took out the ECU in
the fuel injection.  Car had to be towed in.  He replaced the ECU. but not
the alternator, and blew another ECU.  You might want to take the
alternator off and have it tested without driving the van.
Steve Sandlin
'84 Wolfsburg Westy
'79 Transporter