Recently when I am driving the Suburban at around 45-50 if I keep pressure on the gas peddle I hear this weird shutter noise it seems is coming from the back and I am not sure what it is. I checked fluid in rear end and it was a little low so I topped it off but the noise is still there. Any ideas what it could be. Only seems to be around the 45 plus MPH any slower it doesn't seem like it does it. Thanks!