grinding noise under dash? very annoying

[05AZgmc]

GOt about 50k on my 05 gmc ext cab 4x4 1500 and since about when i bought it its had this grinding sort of noise under the dash when i accelerate and bring the rpms up. i took it to the dealer when i was under warrenty and they said they couldnt duplicate the noise, i didnt argue with them cause i didnt have time and couldnt leave it there longer, i have no other way around. anyone else have this problem, a instructor at my school said it might be a quick disconect to the heater core where the coolent goes through and it bubbles there and makes noise, but seems like its a hard part noise, its not the climate control actuator, already got that replaced and its fine so far. any info would be great, thanks.

[05AZgmc]

BTW seems like it might be toward the passenger side of the dash, but not to close to the floor more maybe near the glove box, possibly?