I have a 1993 2500 Suburban, 454 engine with four wheel drive, HD towing package, and 160,000 miles. After a trip to the Nevada desert, I noticed a buzzing/grinding noise coming from what I perceive to be the firewall on the drivers side. The noise is not constant i.e. sometimes it is not present. The noise picks up slightly in intensity on acceleration and decreases when coasting. I first thought the speedometer cable might need cleaning/lubrication because of ther desert pumice dust, but I read somewhere that the speedometer is electric. The cruise control and the master cylinder are in this area, but I cannot hear the noise when the truck is parked. I have driven the truck over 3,000 since this started, and do not notice any other problems other than the power mirror on the drivers side stopped working about the same time. Any ideas?