The procedures suggested above will probably identify your culprit, but if not, you should also try to rule out any possible problem with the electronics/cables chain the TT is feeding (I'll assume you have already ascertained that the problem is not present on any other source besides phono).
You can do this be reversing the leads coming from the TT at the input to the phono preamp. If you reverse the leads, and the problem remains in the same channel, then your distortion is originating somewhere between your phono inputs and your preamp outputs.
However, if you try this and the problem switches channels, but nothing suggested above has helped, then suspect something in the lead-out cables or jacks themselves, with the jacks being the most likely.
You can do this be reversing the leads coming from the TT at the input to the phono preamp. If you reverse the leads, and the problem remains in the same channel, then your distortion is originating somewhere between your phono inputs and your preamp outputs.
However, if you try this and the problem switches channels, but nothing suggested above has helped, then suspect something in the lead-out cables or jacks themselves, with the jacks being the most likely.