Yo other hum seekers and Brianmch:
All phono inputs have a certain residual hiss and hum. This is because phono inputs will have a much higher gain than line input due to low output of magnetic cartridges. CD player or a tuner has about 100 times the output of a magnetic cartridge. Also, phono inputs have a bass boost on them that compensates for the rolled off low frequencies on a record. This makes the phono input much more sensitive to hum, and also makes it generally noisier.
To check residual noise level, unplug the turntable from the phono input set switch to "phono" and set the volume to a normal listening position. That's your residual hum from the amp. Now plug the turntable into the phono input and hook the ground wire, if you have one, to the amp chassis. Be sure to turn volume down all the way before plugging the truntable in or you will get horrifying hum on insertion. Set the volume to normal listening level and that will tell you the residual hum from the turntable.
In this case, since it may be mechanical noises unrelated to amplifier, also plug the turntable into the wall but NOT into the amp and see if it still makes the noise when you turn it on and operate it with it not plugged into amp. If it does, your sound is a mechanical resonance phenomenon. You can locate it by using the vacuum hose stethoscope like I described, or by touching different spots on turntable and seeing if the sound changes when you touch things. The thing that changes the sound when you touch it is the problem item.
The test where you lay the arm on a stopped turntable and advance the volume tests for turntable isolation and feedback. If you get an increasing hum or rumble when you advance the volume to normal listening level or beyond, you need to get the turntable away from the speakers and isolate it from speakers. If your problem is really an electronic hum problem, then feedback, if present, will make it worse.
First, you need to identify the source of your problem. And that's all for what it's worth. And good luck from the old diagnostician,