Were you using a SUT with your MC cartridges going into the MM input on your phono stage? Likely you have a bad ground between your TT and your preamp, which is now loaded at 47k...not the much lower load seen by your SUT, which also acts to isolate your TT. This is a common problem when moving from low impedance to high impedance cartridges...your system is now revealing all your grounding issues.
Make sure your tone arm is grounded, and you have a ground wire running from the TT to your phono stage. If turning the TT motor on and off makes a diff, try lifting the ground on the motor or your phono preamp.
Make sure your tone arm is grounded, and you have a ground wire running from the TT to your phono stage. If turning the TT motor on and off makes a diff, try lifting the ground on the motor or your phono preamp.