Anyone else hearing a vibrating/growling-type noise in between 40 & 50 mph? I took my tahoe to the dealer once over the problem and they said it was the baffles in the muffler, so they replaced the muffler...the noise quietened down for a while and now it's back and loud as ever. I took it back today for another problem as well as that noise and they assured me it was "normal" and all tahoes/suburbans do it. I know there is a growling-type noise which IS in fact normal at certain speeds or when you're in a certain gear, but this noise is most def not that! It's way too loud and funny sounding to be normal. Help?