I have it both accelerating and over rough roads. The rough roads part occurs during braking, accelerating (worse then) and when at a constant speed.
If it truly is a relay. It sounds like it may have a week coil or spring to hold the contacts open or closed (read relay's normal postion while driving). If we knew the location we might be able to adjust the mount to lessen the noise or find a replacemnt with stronger springs/coil... Just my two cents and if you can get any extra info I would be willing to give things a try.
Smoke 04 Titan LE, Big Tow, Splash guards, Bed Extender, Leaf Spring Helpers, Hellwig Anti-Sway bar, Mobil 1, Xaino, Born on 04/26/2004
MB Quart front and back, Phoenix Gold R9.0.5, Custom iPod install