480 CE with white vapor – cause?

I have 2005 Cummins 480CE’s in a Sea Ray. Last year my port engine began developing what can be best described as a slight white vapor or smoke which only appears at cruise speed at 2400 RPM. If I back off to a lower rpm the vapor disappears. I’ve owned boat for seven years and only last year began to notice this occur. About 800 hours on the motors. I’ve had the engines serviced, replacing all fluids, had the after coolers serviced and rebuilt this past winter, (were in great condition I was told) SBMAR water pumps serviced with new impellers, all new air, oil, and fuel filters, and just about any other preventive maintenance one could do.
Removed and inspected elbows for obstructions but all found clean. No water found in coolant or oil. Oil test came back fine. Engine runs strong and temp averages normal at 165-170. Only other thing done at start of last season was the replacement of the two water exhaust hoses which attach to thru-hull fitting as they were old and deteriorating. Despite all that has been done the white vapor/smoke remains. (It’s very light.) ??? Thoughts appreciated. Thanks in advance.