GMC Troubleshooting Questions? Ask a Mechanic for Answers
Hi, I am Mike, welcome. I'm reviewing your question.
If it doesn't change with the rpm, that it appears it is not coming from the engine.
What did the dealer say it was specifically?
If he cannot or will not tell you, I would suspect him of fabricating a response.
If you could possibly make an audio recording of it and post it so I can hear it, I ight be able to help further.