Turn every thing off and then unplug each lead and give all the connectors a clean with a fine emery board or wire wool. Do the same with the cartridge - very carefully. My turntable also has a separate earth lead that is connected to one of the outer braided signal leads, if yours has clean that connection as well. If it uses a different mains socket to the main amp try plugging them into the same socket if possible.
Mu-metal is what you want from magnetic shielding but I don't know where it is available now days.