Rattle/knocking at speed

[022520]

Hi,

Recently began hearing a knocking coming from the passenger side toward the rear of my 2001 base box. The weird part is that the sound usually appears at about 50 mph. The really weird part is that I can make the sound go away by further accelerating but it comes back when I take my foot off the gas. Any ideas? Have about 115,000 miles.

Thanks, Larry