Valve Squeal

[E Sully]

I have an '86 3.2 with about 65,000 miles in my 73.5 911T. I had a reputable shop that I have known for over 30 years install new valve guides and new exhaust valves. Intake valves were still good. New rings were also installed, Nikasil cylinders and pistons up to spec. Everything on the top end was carefully assembled following Waynes book.
Started up fine and was @15 minutes into the 20 minute recommended run. Oil temp and pressure were good. Suddenly there was a squealing sound. Shut it down right away.
Engine was pulled and put on a stand. I took off the valve covers and turned the engine with the pulley bolt. I was able to isolate the squeal to the #5 exhaust valve. As the cam lobe started closing the valve I get a distinct noise. I pulled the rocker arm and checked again. Noise goes away. Reinstalled rocker arm and noise is back. I put a couple of drops of WD-40 on the valve stem and it stopped squealing. The heads are currently back at the shop where he will check everything out when he gets a chance. Oil pressure and temp were good and everything else, looks great. Any ideas what might be causing this?