I agree with @audphile. One of the small clips wired to the cartridge pins could be making poor connection.
If you have a detachable headshell, there could be poor contact between the pins on the headshell and the tonearm connection. This has happened to me.
The shield of one or more cables might have been disconnected internally. You will need an ohmmeter to check this or you could inspect visually. Remove the cap on the RCA connector (screws off) and visually check connection. You could gently prod the wires to see that they are firmly connected to the plug.
It would be good to check out the all connections from the cartridge pins all the way.to the input of the phono preamp.
You can also test each cable independently by replacing one at a time and checking for noise.
Also check that the ground on the phono cable is firmly connected to the phono preamp ground. Break and remake connections.
Of course, do not have phono preamp or line level preamp power on while making changes. Turn power on only after all connections are made and checked.
Happy troubleshooting!