2003 Engine Droan

My '03 2500HD Has a loud resonating engine noise which, when it occurs, happens between 48-54 mph and little to no acceleration. I can power through the noise to make it go away or usually around 55 + mph it will go away. This is an intermittent problem, but it is quite loud and annoying when it occurs. Had it in to the dealer w/about 700 miles on it. Dealer said (and showed me a GM service bulletin), it was an inexplicable problem that GM is having w/several of the '03 P/U and SUV type vehicles. The other day I heard, on radio news, that GM is buying back some vehicles that have a curious engine knock. Is this the same problem? I can't find any information about the buyback. Anyone else have this problem?

My '03 Z71 has a loud groan in the passenger compartment when I have the A/C on. It is most heard at about 50-65 mph.
Dean