So I was thinkin this issue over last night, couldn't get it out of my head. Then it hit me, I had the same problem on a V8 Ranger project years back. I miss that truck, mustang eater it was... ;-)

Water pump vanes had slipped loose from the shaft. They would only move/spin as rpm increased. Think plastic fan blades spinning freely on motor shaft. No leaks whatsoever but no flow either.

 

so what was the culprit?

 

Everything seems to be good still. So I'm thinking the problem was a nasty air-lock. after 4 weeks and 1400 miles, all gages are still normal, the coolant level hasn't moved in the jug. The faint high pitch sound I thought I heard hasn't been back.

 

also it would prob be a good idea to flush out your radiatior IMO...it might solve the prob...did it to mine the other day and its runnin a whole lot better