When I accelerate my vehicle, usually onto the freeway, but sometimes on a fast street, at about 40 MPH, it will make this rumble noise like I was going over those drunk bumps on the highway. It might last about two seconds and happen 7 or 8 times and seems to happen at about 40 -60 MPH. If I accelerate quickly or drive at 65+MPH, it does not seem to happen. It does not seem like my car is slowed by this, nor are there any steering problems, but it is annoying and it simply should not happen. The RPM also rises about 50-100 every time the drunk bump noise happens - the same even when they don't. If the car is warmed up after 15 miles or so, it mostly ceases, but it is not often I drive that far commuting. Any ideas? Thanks! Andy 1999 suburban 4x4 350