4WD noise?

I have an '02 tahoe 4WD n 4HI, it makes a kinda grinding, bad bearing noise. I took it in and they said they drove it, and heard the noise. Up on the rack, it did not. They compared it to an '01 Tahoe 4WD and they said it did the same thing. Anyone have this same problem?

Also I asked about the "piston slap" and they also said its normal. Do we know if this is correct? And if it is, have engines failed because of it?

Any info will help, thanks.