Looking for help:

I have a 99 GMC Sierra x-cab 4wd and for about the past month, I have been hearing/feeling a rumbling or soft grinding noise when I reach about 38 to 40 mph and slowly press on the gas. Details are as follows: It seems like the engine has to be at or above 180 degrees (for the overdrive/tourque converter to kick in). I can be going 35 and it won't do it until it downshifts to whatever gear it is when I hit about 38 to 40 (4th??). Then when I am going about 40 consistently (about 11 or 1200 rpms) and I very lightly press on the gas, then I hear the noise. I have never heard before about a month ago. The truck has about 85k on it. I took it to the shop and had a guy ride with me, but he claimed it was just the tourque converter I was hearing once it reached that gear and speed. I am no mechanic an don't pretend to be, but I never heard it before and to me, it's not "normal" as the mechanic said. Anyone have any ideas?

Thanks.

CHeck the belt tensioner to be sure it isn't "bottomed-out". A loose belt or a faulty tensioner can cause this condition. Itcould also be an exhaust heat shield. I've seen both happen frequently on GM's.