Turntable motor question help needed please !

Need help in resolving a problem. I own two turntables, a Marantz TT 15is (made by Clearaudio) and a Clearaudio Emotion. 

Both have the same Clearaudio motor that sits in a hole cavity, not touching the turntable.

Lately, I have been getting this intermittent rumble sound from the motor on the Emotion. It is very audible.

While the motor is still running, if I apply a small pressure with a fingernail on top at the very centre of the spindle wheel, the noise-rumble diseapears momentarily, only to come back after a few minutes, sometimes a few seconds. Is there any lubrification needed to remove whatever causes this friction or is this non-advisable for fear of damaging the motor?

My shelf is totally level for your information.

Thanks if you can help!