silverado noise

This letter is in conjunction with my last one. I made a typo when I said the service advisor could make out where the noise comes from. He could not. He thought maybe something may be rubbing or maybe noise being transferred from a rubber bushing in the front end . To me it seems to be coming from around the heater core area as when we took the truck for a drive he unhooked the glove box so it hung down and the noise was louder this way. I forgot to mention that the noise is more pronounced when I tow my trailer and the revs go up. Noise comes in at around 2300 rpms and gets louder as the revs go up and stops when revs drop.
Chris e