Soot in overflow tank

[miker61002]

I had a leaky EGR cooler, and I was going through about 1/2 to 2/3 gallon of coolant per day (roughly 500-600 miles). The EGR valve was also replaced. Flushed the system and recharged. The mechanic noted soot in the overflow tank, and, even though there are NO external leaks visible, I'm still using coolant. Does anyone know what my issue could be?

The truck SEEMS to be pulling O.K., but, under 30 psi or so boost, I can hear a slight whine (kind of like the ignition noise 4-wheelers in the 70's had). The oil doesn't show any coolant contamination (by sight or smell).

Engine data No: 79229273 Family: 6CEXH0912XAH CPL 8519

TIA.

Michael