Thanks, guys, for all your responses. Indeed, the ground wire was broken; I spliced it and 95% of the noise is gone. Certainly not as quiet as digital, which is practically noise-free, but quite an improvement!
Is analog supposed to be so noisy?
I am experimenting with a Luxman TT which is actually in pretty good shape. My system consists of the Prima Luna Prologue pre-amp and the Prologue 6 monoblocks. A phono input was installed in the preamp when I bought the units. As soon as I turn the knob to the phono input, I am bombarded with a fairly significant hum, which is not present when I use the settings for my CD players. Is this something normal, or am I doing something wrong? By the way, I have also tried my old Moscode Minuet in A, which includes a grounding plug for the turntable, and I am still getting the same problem.