2001 Dodge Ram rumble sounds over 40mph when hitting bumps

When im driving over 40mph and I hit a bump I get a vibration it sounds like when u go over the rumble strips in the road the faster I go the louder it gets but when I hit a bump again it stops this can go on and on if im close to 40mph and I let off the gas and I reach below 40mph it stops I never get the noise under 40 no matter how many bumps I hit can someone let me know wat this is oh I already replaced the u joint in the left side front axle that controlls the 4x4 my mechanic now wants to check everything out at my cost for the labor thank you