I was having that issue with my 96. And it ended up being the body mount bolts. The cab was moving enough to slightly knock against an engine component. One of the bolts had worked it's way out and the bushings were deteriorated. I got under her and rattled all the cab mount bolts out. I suggest an air impact, a breaker bar and cheater, and a couple cans of penetrating lubricant. I used a hole saw and made rubber bushings outta an old tire and mounted those between the existing and new cab bushing and the frame. You'd have to get some longer mounting bolts but it worked for me.