Trouble tracking down a noise....need help please

I am get a rumbling when I am driving at 50 or above. The minute I let go of the gas or turn a little to the left I get a non stop rumble. A light tapping noise. I can feel it at the floor board. Any ideas? It goes away if
I step on the gas or the brake. I don't think it is the brake because it only does it at high speeds. Any ideas? Thanks

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