Try unplugging the preamp and the turntable from the amp/receiver. Then turn it way up. This could be some organic failure within your amp/receiver. I agree with jdjohn that it is not likely a grounding problem. These usually show up as hums or buzzes.
I also agree that you should see if you have added something new or changes something about the system. If you detach the ground wire from the phono preamp, you should get a change, adding either a hum or a buzz.
For the h**l of it, try attaching the turntable ground wire directly to the amp/receiver chassis directly. The fact that the noise comes and goes suggests that it is caused by something loose. Check your phono plugs where they plug into the amp/receiver, and squeeze the outside ring on the phono plug to be sure that it grips the jack tightly.
And good luck from the old "try this" experimenter,