Curious about this...Sounds like what mine is doing but only from time to time at about 11-1400 rpm.....Have extended powertrain warranty through GM and I am going to have to get it in to see if in fact its the water pump going. Replaced both idlers and tensioner...that would have been a cheap fix but not to be.
2005 LLY Chevrolet 2500HD
4 Door Crew, LT, BD Primary Fuel Filter (Cat 1R-0749),
AMSOIL Engine Oil Bypass, Raptor RP-100 Lift Pump
Dieselsite Bypass Coolant Filter
AFE Mouthpiece, Firestone Air Ride,
Undercover Tonneau Cover, Bed Rug,
223,000 kms. and counting