Do you have another cart you can swap in? Grados are known as being particularly sensitive to picking up noise and hum.
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It sounds like the tonearm ground has a poor, or broken connection somewhere, is the most likely thing I can think of.Try wrapping some soft wire strands (after stripping 2-3 inches of insulation off it)around the arm, with enough length of wire to ground the other end. Some flexible light gauge stranded wire. Maybe just tape over the strands of wire on the arm wand so it doesn't cause any scratches on it. If this works, that would verify that there may be a poor connection on the arm ground, maybe where its out of sight.
Also, if your arm has a "SME" type of plug in headshell, maybe try running a ground from the cartridge leads ground wires, maybe off of a clip ground to the metal headshell, if it is metal, otherwise this idea wouldn't do anything. If it does work, double check to make sure you don't loose anything sonically. There are different kinds of grounding schemes used in some arms. I've seem some use a separate shield over the lead wires going through the arm. This was done in metal arms too, not just non-metallic arms. Also a little metal foil in between the headshell and cartridge, grounded to to cartridge ground, if the headshell is non-metallic.
Also, if your arm has a "SME" type of plug in headshell, maybe try running a ground from the cartridge leads ground wires, maybe off of a clip ground to the metal headshell, if it is metalThis may also work for any metal of head mount, as long as it is metal, and bonded/connected to the metal arm wand.