Is the U relay the one that you replaced? If so then you can rule out the coil as being the problem as such. I feel you need to do what Retropin suggested above -:
Assuming the relay's coil has proper resistance and is not burnt, there are a couple things that can be done to minimize the buzz/hum. Adjust the relay with as little spring resistance as possible to pull in the armature plate. This will minimize the noise. Adjust the relay's switch blades and lengthen the spring so only the minimal amount of tension is needed to return the armature plate back to it's resting position. Even the contact surface between the relay coil and the activator plate can be filed to smooth it if it has wear. (Or just replace the activator plate and/or relay.)
