I have a 2003 GMC 2500HD 6.0liter vortech 4speed automatic. I was going down a steep grade and the transmission started making a really loud grindy whindy noise and then stops when you put gas on or slow right down. at first I thought it was an engine brake but the truck actually sped up a little when the noise happened. DO you know what this is?

SOURCE: MAP SENSOR 93 GMC SIERRA 1500 2wd

youare dealing with a spark plug or sparkplug wire thats breaking down causeing a mist thats what your surge is i ypuld change both

SOURCE: 2002 gmc sierra truck won't start replaced filter

Both vehicles?I would suspect bad fuel,water contaminated possibly?

SOURCE: 2003 GMC SIERRA 2500HD

I have the same issue...what's the fix?

SOURCE: Loud Ticking/knocking Noise From motor; What is it?????

You have a hydraylic lifter bleeding down,they make some good oil additives that should help.

SOURCE: 2004 GMC Yukon Denali Awd humming sound in front of truck

The Denali's are really susceptible to front dif. wear and failure. Because it is AWD, the front dif is always working unlike a 4WD. GM, in order to lighten up the front end built the front dif in aluminum. If it was the same as the rear, it would be bullet proof, but much heavier. After about 80,000 miles it is common to have bearings failing. Remove the drain plug which happens to be magnetic. You may see more than just the normal fine filings. You will likely see larger chunks of metal some of which is not even magnetic but will drain out with the oil. You are likely looking at a $1500 rebuild. It will get much louder and run for a long time before it really goes south.